0000950170-21-002819 | 10-Q | XBRL Viewer

Cash Equivalents and Marketable Securities - Additional Information (Detail)	9 Months Ended
Sep. 30, 2021
Cash equivalents and marketable securities [Line Items]
Marketable securities contractual maturities period	1 year
Realized gains or losses on marketable securities, description	no material realized gains or losses

Label	Element	Value
Receivables Gross	grts_ReceivablesGross	$ 0
Receivables Gross	grts_ReceivablesGross	0
Receivables Gross	grts_ReceivablesGross	2,100,000
Receivables Gross	grts_ReceivablesGross	0
Payments For Options To License For Each Indication	grts_PaymentsForOptionsToLicenseForEachIndication	1,500,000
Accrued Research And Development Expense Current	grts_AccruedResearchAndDevelopmentExpenseCurrent	2,540,000
Accrued Research And Development Expense Current	grts_AccruedResearchAndDevelopmentExpenseCurrent	$ 1,053,000
Lessee Leasing Arrangements Operating Leases Term Of Contract Month And Year	grts_LesseeLeasingArrangementsOperatingLeasesTermOfContractMonthAndYear	2025-04
Lessee Leasing Arrangements Operating Leases Term Of Contract Month And Year	grts_LesseeLeasingArrangementsOperatingLeasesTermOfContractMonthAndYear	2023-01
Issuance of common stock for warrant exercises, Share	grts_IssuanceOfCommonStockForWarrantExercisesShare	9,555,876
Issuance of common stock for warrant exercises, Share	grts_IssuanceOfCommonStockForWarrantExercisesShare	9,555,876
Issuance of common stock for warrant exercises, Share	grts_IssuanceOfCommonStockForWarrantExercisesShare	9,555,876
Issuance of common stock for warrant exercises, Share	grts_IssuanceOfCommonStockForWarrantExercisesShare	9,555,876
Amount To Extend Option Term	grts_AmountToExtendOptionTerm	$ 1,000,000.0
Net Contract Asset	grts_NetContractAsset	1,096,000
Net Contract Asset	grts_NetContractAsset	0
Stock Issued During Period Value Purchase Agreement	grts_StockIssuedDuringPeriodValuePurchaseAgreement	20,830,000
Stock Issued During Period Value Purchase Agreement	grts_StockIssuedDuringPeriodValuePurchaseAgreement	20,830,000
Maximum Aggregate Contingent Milestone Payment To Be Made	grts_MaximumAggregateContingentMilestonePaymentToBeMade	191,000,000.0
Collaboration Receivable	grts_CollaborationReceivable	2,132,000
Collaboration Receivable	grts_CollaborationReceivable	0
Upfront Payments	grts_UpfrontPayments	1,500,000
Upfront Payments	grts_UpfrontPayments	1,500,000
Lessee Operating Lease Liability Payments Due After Year Four	grts_LesseeOperatingLeaseLiabilityPaymentsDueAfterYearFour	$ 69,777,000
Lessee Leasing Arrangements Operating Leases Additional Number Of Renewal Terms	grts_LesseeLeasingArrangementsOperatingLeasesAdditionalNumberOfRenewalTerms	2
Lessee Leasing Arrangements Operating Leases Additional Number Of Renewal Terms	grts_LesseeLeasingArrangementsOperatingLeasesAdditionalNumberOfRenewalTerms	2
Amount Of Irrevocable Letters Of Credit Withdrawn	grts_AmountOfIrrevocableLettersOfCreditWithdrawn	$ 0
Amount Of Irrevocable Letters Of Credit Withdrawn	grts_AmountOfIrrevocableLettersOfCreditWithdrawn	0
Milestone Payments	grts_MilestonePayments	1,000,000.0
Milestone Payments	grts_MilestonePayments	1,000,000.0
Operating lease payment under the boston lease	grts_OperatingLeasePaymentUnderTheBostonLease	79,113,000
Issuance of common stock for warrant exercises, Value	grts_IssuanceOfCommonStockForWarrantExercisesValue	1,000
Issuance of common stock for warrant exercises, Value	grts_IssuanceOfCommonStockForWarrantExercisesValue	39,000
Issuance of common stock for warrant exercises, Value	grts_IssuanceOfCommonStockForWarrantExercisesValue	40,000
Issuance of common stock for warrant exercises, Value	grts_IssuanceOfCommonStockForWarrantExercisesValue	1,000
Issuance of common stock for warrant exercises, Value	grts_IssuanceOfCommonStockForWarrantExercisesValue	39,000
Issuance of common stock for warrant exercises, Value	grts_IssuanceOfCommonStockForWarrantExercisesValue	$ 40,000
Operations expected commenced date	grts_OperationsExpectedCommencedDate	2022-11
Initial funding development costs	grts_InitialFundingDevelopmentCosts	$ 20,600,000
Prepaid Expense For Research And Development	grts_PrepaidExpenseForResearchAndDevelopment	3,480,000
Prepaid Expense For Research And Development	grts_PrepaidExpenseForResearchAndDevelopment	$ 2,584,000
Lease expiration term	grts_LeaseExpirationTerm	2033-01
Stock Issued During Period Shares Purchase Agreement	grts_StockIssuedDuringPeriodSharesPurchaseAgreement	1,169,591
Restricted cash deposit	grts_RestrictedCashDeposit	$ 4,700,000
Proceeds From Non Refundable Upfront Payment	grts_ProceedsFromNonRefundableUpfrontPayment	20,000,000.0
Proceeds From Non Refundable Upfront Payment	grts_ProceedsFromNonRefundableUpfrontPayment	30,000,000.0
First Tranche of Fundings Received	grts_FirstTrancheOfFundingsReceived	11,300,000
Contract With Customer Liability Revenue Recognized Including New Contracts In Period	grts_ContractWithCustomerLiabilityRevenueRecognizedIncludingNewContractsInPeriod	0
Contract With Customer Liability Revenue Recognized Including New Contracts In Period	grts_ContractWithCustomerLiabilityRevenueRecognizedIncludingNewContractsInPeriod	38,600,000
Contract With Customer Liability Revenue Recognized Including New Contracts In Period	grts_ContractWithCustomerLiabilityRevenueRecognizedIncludingNewContractsInPeriod	1,600,000
Contract With Customer Liability Revenue Recognized Including New Contracts In Period	grts_ContractWithCustomerLiabilityRevenueRecognizedIncludingNewContractsInPeriod	4,100,000
Contract With Customer Liability Revenue Recognized Including New Contracts In Period	grts_ContractWithCustomerLiabilityRevenueRecognizedIncludingNewContractsInPeriod	200,000
Maximum Securities Covered Under Registration Agreement	grts_MaximumSecuritiesCoveredUnderRegistrationAgreement	250,000,000.0
Payments Of Stock Issuance Costs Under Sales Purchase Agreement	grts_PaymentsOfStockIssuanceCostsUnderSalesPurchaseAgreement	$ 339,000
Collaboration Agreement Early Termination Notice Period	grts_CollaborationAgreementEarlyTerminationNoticePeriod	120 days
Collaboration Agreement Early Termination Notice Period	grts_CollaborationAgreementEarlyTerminationNoticePeriod	90 days
Non Refundable Option Fee	grts_NonRefundableOptionFee	$ 40,000,000.0
Maximum Aggregate Contingent Milestone Payment	grts_MaximumAggregateContingentMilestonePayment	685,000,000.0
Contract With Customer Liability Addition	grts_ContractWithCustomerLiabilityAddition	0
Contract With Customer Liability Addition	grts_ContractWithCustomerLiabilityAddition	220,000
Contract With Customer Liability Addition	grts_ContractWithCustomerLiabilityAddition	11,282,000
Adjustments To Additional Paid In Capital Offering Costs Related To Sale Of Common Stock And Pre Funded Warrants	grts_AdjustmentsToAdditionalPaidInCapitalOfferingCostsRelatedToSaleOfCommonStockAndPreFundedWarrants	(451,000)
Adjustments To Additional Paid In Capital Offering Costs Related To Sale Of Common Stock And Pre Funded Warrants	grts_AdjustmentsToAdditionalPaidInCapitalOfferingCostsRelatedToSaleOfCommonStockAndPreFundedWarrants	(451,000)
Increase Decrease In Deposit And Other Assets Noncurrent	grts_IncreaseDecreaseInDepositAndOtherAssetsNoncurrent	327,000
Increase Decrease In Deposit And Other Assets Noncurrent	grts_IncreaseDecreaseInDepositAndOtherAssetsNoncurrent	$ (29,000)
Early Termination Reduction Of Future Milestone And Royalty Payments	grts_EarlyTerminationReductionOfFutureMilestoneAndRoyaltyPayments	Additionally, all of bluebird’s payment obligations that have not yet accrued related to future milestone and royalty payments will be reduced by 50% for the remainder of the agreement term.
Early Termination Reduction Of Future Milestone And Royalty Payments	grts_EarlyTerminationReductionOfFutureMilestoneAndRoyaltyPayments	Additionally, if terminated early by Gilead for convenience or by the Company for material breach or insolvency, all of Gilead’s payment obligations for reimbursable costs or for future milestone and royalty payments remain. If terminated early by Gilead for material breach or insolvency, all of Gilead’s unaccrued payment obligations related to future milestone and royalty payments will be reduced by 50% for the remainder of the agreement term.
Contract With Customer Liability Deductions	grts_ContractWithCustomerLiabilityDeductions	$ (2,130,000)
Contract With Customer Liability Deductions	grts_ContractWithCustomerLiabilityDeductions	(220,000)
Contract With Customer Liability Deductions	grts_ContractWithCustomerLiabilityDeductions	(287,000)
Maximum Offering Expenses Agreed To Be Reimbursed	grts_MaximumOfferingExpensesAgreedToBeReimbursed	$ 50,000
Collaboration Agreement Termination Term	grts_CollaborationAgreementTerminationTerm	Gilead may terminate the Gilead Supply Agreement without cause by giving six months prior written notice and any active orders with 60-day notice without terminating the agreement, and either party may terminate based on an uncured material breach, insolvency of the other party, or in the event that the Gilead Collaboration Agreement is terminated. Upon termination, the Company will deliver all supply products that have been produced and destroy, reimburse or deliver materials that Gilead has reimbursed, and Gilead must pay for any manufacturing costs that the Company has actually incurred or committed to pay, including any cancellation costs owed to subcontractors.
Transaction Price Allocated To Shares Of Common Stock At Fair Value	grts_TransactionPriceAllocatedToSharesOfCommonStockAtFairValue	$ 21,200,000
Contract With Customer Asset Deduction	grts_ContractWithCustomerAssetDeduction	$ (2,164,000)
Share Based Compensation Arrangement By Share Based Payment Award Number Of Shares Available For Grants Exercised	grts_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAvailableForGrantsExercised	0
Interest And Other Receivables Current	grts_InterestAndOtherReceivablesCurrent	$ 196,000
Interest And Other Receivables Current	grts_InterestAndOtherReceivablesCurrent	8,000
Warrants issued	grts_WarrantsIssued	0
Increase Decrease In Accrued And Other Liabilities Non Current	grts_IncreaseDecreaseInAccruedAndOtherLiabilitiesNonCurrent	106,000
Increase Decrease In Accrued And Other Liabilities Non Current	grts_IncreaseDecreaseInAccruedAndOtherLiabilitiesNonCurrent	1,041,000
Warrants outstanding	grts_WarrantsOutstanding	0
Increase Decrease In Accrued Research And Development Expenses	grts_IncreaseDecreaseInAccruedResearchAndDevelopmentExpenses	1,487,000
Increase Decrease In Accrued Research And Development Expenses	grts_IncreaseDecreaseInAccruedResearchAndDevelopmentExpenses	$ 778,000
Revenue Recognition Contract End Month Year	grts_RevenueRecognitionContractEndMonthYear	2023-08
Common Stock Purchase Price In Excess Of Fair Value1	grts_CommonStockPurchasePriceInExcessOfFairValue1	$ 8,800,000
Warrants To Purchase Shares Of Common Stock	grts_WarrantsToPurchaseSharesOfCommonStock	27,480,719
Use Of Net Operating Losses Suspension Period For Medium And Large Business	grts_UseOfNetOperatingLossesSuspensionPeriodForMediumAndLargeBusiness	3 years
Increase Decrease In Lease Liability	grts_IncreaseDecreaseInLeaseLiability	$ (5,902,000)
Increase Decrease In Lease Liability	grts_IncreaseDecreaseInLeaseLiability	(2,512,000)
Minimum Amount Of Taxable Income Where Use Of Net Operating Losses Suspended	grts_MinimumAmountOfTaxableIncomeWhereUseOfNetOperatingLossesSuspended	$ 1,000,000.0
Revenue Recognition Contract Start Month Year	grts_RevenueRecognitionContractStartMonthYear	2018-08
Revenue Recognition Contract Start Month Year	grts_RevenueRecognitionContractStartMonthYear	2021-01
Revenue Recognition Contract End Period	grts_RevenueRecognitionContractEndPeriod	2 years
Revenue Recognition Contract End Period	grts_RevenueRecognitionContractEndPeriod	4 years
Lapse Of Repurchase Rights Related To Common Stock Issued Pursuant To Early Exercises Shares	grts_LapseOfRepurchaseRightsRelatedToCommonStockIssuedPursuantToEarlyExercisesShares	30,874
Use Of Business Incentive Tax Credits Period Cap	grts_UseOfBusinessIncentiveTaxCreditsPeriodCap	3 years
Maximum Milestone Payment Obligation	grts_MaximumMilestonePaymentObligation	$ 36,400,000
Maximum Milestone Payment Obligation	grts_MaximumMilestonePaymentObligation	34,800,000
Lapse Of Repurchase Rights Related To Common Stock Issued Pursuant To Early Exercises	grts_LapseOfRepurchaseRightsRelatedToCommonStockIssuedPursuantToEarlyExercises	11,000
Lapse Of Repurchase Rights Related To Common Stock Issued Pursuant To Early Exercises	grts_LapseOfRepurchaseRightsRelatedToCommonStockIssuedPursuantToEarlyExercises	11,000
Maximum Potential Milestone Receivable	grts_MaximumPotentialMilestoneReceivable	1,200,000,000
Proceeds From Issuance Of Common Stock And Pre Funded Warrants	grts_ProceedsFromIssuanceOfCommonStockAndPreFundedWarrants	119,800,000
Cash Cash Equivalents And Marketable Securities	grts_CashCashEquivalentsAndMarketableSecurities	199,700,000
Proceeds From Maturity Of Marketable Securities	grts_ProceedsFromMaturityOfMarketableSecurities	59,390,000
Proceeds From Maturity Of Marketable Securities	grts_ProceedsFromMaturityOfMarketableSecurities	60,982,000
Proceeds From Sale Of Marketable Securities	grts_ProceedsFromSaleOfMarketableSecurities	6,225,000
Proceeds From Sale Of Marketable Securities	grts_ProceedsFromSaleOfMarketableSecurities	5,401,000
Maximum Milestone Consideration Payable	grts_MaximumMilestoneConsiderationPayable	123,500,000
Agreed Upfront Payment	grts_AgreedUpfrontPayment	2,000,000.0
Agreed Upfront Payment	grts_AgreedUpfrontPayment	$ 2,000,000.0
Class Of Warrant Or Right Issue Date	grts_ClassOfWarrantOrRightIssueDate	Dec. 28, 2020
Milestone Payment Included In Research And Development Expense	grts_MilestonePaymentIncludedInResearchAndDevelopmentExpense	$ 2,500,000
Milestone Payment Included In Research And Development Expense	grts_MilestonePaymentIncludedInResearchAndDevelopmentExpense	$ 3,000,000.0
Class Of Warrant Or Rights Expiration	grts_ClassOfWarrantOrRightsExpiration	None
Remeasurement Of Operating Lease Right Of Use Assets For Lease Modification	grts_RemeasurementOfOperatingLeaseRightOfUseAssetsForLeaseModification	$ 6,562,000
Remeasurement Of Operating Lease Right Of Use Assets For Lease Modification	grts_RemeasurementOfOperatingLeaseRightOfUseAssetsForLeaseModification	$ 3,174,000
Class Of Warrant Or Right Warrants Exercised	grts_ClassOfWarrantOrRightWarrantsExercised	9,560,748
Class Of Warrant Or Right Warrants Exercised	grts_ClassOfWarrantOrRightWarrantsExercised	9,560,748
Contract With Customer Asset Addition	grts_ContractWithCustomerAssetAddition	$ 3,260,000
Aggregate Payment In Specified Development Regulatory And Commercial Milestone	grts_AggregatePaymentInSpecifiedDevelopmentRegulatoryAndCommercialMilestone	$ 71,000,000.0

Balance Sheet Components - Schedule of Deposits and Other Long-Term Assets (Detail) - USD ($) $ in Thousands	Sep. 30, 2021	Dec. 31, 2020
Deposits And Other Long Term Assets Disclosure [Abstract]
Prepaid research and development-related expenses	$ 521	$ 518
Total deposits and other long-term assets	$ 2,064	$ 1,736

Cash Equivalents and Marketable Securities - Schedule of Amortized Cost, Unrealized Gains and Losses and Fair Value of Cash Equivalent and Marketable Securities (Detail) - USD ($) $ in Thousands	Sep. 30, 2021	Dec. 31, 2020
Cash equivalents and marketable securities [Line Items]
Unrealized Gains	$ 0	$ 0
Unrealized Losses	0	0
Amortized Cost	134,703	37,803
Unrealized Gains	11	0
Unrealized Losses	(4)	0
Fair Value	$ 134,710	$ 37,803