UNITED STATES

SECURITIES AND EXCHANGE COMMISSION

Washington, D.C. 20549

SCHEDULE 13D

Under the Securities Exchange Act of 1934

(Amendment No. 1)*

Gritstone Oncology, Inc.

(Name of Issuer)

Common Stock, $.0001 par value per share

(Title of Class of Securities)

39868T105

(CUSIP Number)

Robert Liptak

Clarus Ventures, LLC

101 Main Street, Suite 1210

Cambridge, MA 02142

(617) 949-2200

(Name, Address and Telephone Number of Person Authorized to Receive Notices and Communications)

January 4, 2019

(Date of Event which Requires Filing of this Statement)

If the filing person has previously filed a statement on Schedule 13G to report the acquisition that is the subject of this Schedule 13D, and is filing this schedule because of §§240.13d-1(e), 240.13d-1(f) or 240.13d-1(g), check the following box.  ☐

Note: Schedules filed in paper format shall include a signed original and five copies of the schedule, including all exhibits. See §240.13d-7 for other parties to whom copies are to be sent.

The information required on the remainder of this cover page shall not be deemed to be “filed” for the purpose of Section 18 of the Securities Exchange Act of 1934 (“Act”) or otherwise subject to the liabilities of that section of the Act but shall be subject to all other provisions of the Act (however, see the Notes).

Schedule 13D

This Amendment No. 1 (“Amendment No. 1”) to the statement on Schedule 13D amends and supplements the statement on Schedule 13D originally filed on October 12, 2018 (the “Original 13D”) relating to the Common Stock, par value $0.0001 per share (the “Common Stock”), of Gritstone Oncology, Inc. (the “Issuer”). The address of the principal executive offices of the Issuer is 5858 Horton Street, Suite 210, Emeryville, CA 94608.

This Amendment No. 1 is being filed in connection with the acquisition by The Blackstone Group L.P. and certain of its affiliates (“Blackstone”) of Clarus Ventures, LLC and certain of its affiliates (collectively, “Clarus”). On January 4, 2019, Blackstone integrated the Clarus business into Blackstone (the “Clarus Integration”). As a result of such integration, Blackstone may be deemed to have become the beneficial owner of the securities beneficially owned by Clarus, including the securities of the Issuer.

Certain terms used but not defined in this Amendment No. 1 have the meanings assigned thereto in the Schedule 13D. Except as specifically provided herein, this Amendment No. 1 does not modify any of the information previously reported on the Original 13D.

This statement is being filed by:

(a) Clarus Lifesciences III, L.P. (the “Fund”);

(b) Clarus Ventures III GP, L.P. (“Clarus GP”), which is the sole general partner of the Fund;

(c) Blackstone Clarus III L.L.C., which is the sole general partner of Clarus GP, Blackstone Holdings II L.P., which is the sole member of Blackstone Clarus III L.L.C., Blackstone Holdings I/II GP Inc., which is the sole general partner of Blackstone Holdings II L.P., The Blackstone Group L.P., which is the controlling shareholder of Blackstone Holdings I/II GP Inc., and Blackstone Group Management L.L.C. (collectively, with Blackstone Clarus III L.L.C., Blackstone Holdings II L.P., Blackstone Holdings I/II GP Inc. and The Blackstone Group L.P., the “Control Entities”), which is the sole general partner of The Blackstone Group L.P. and which is wholly-owned by Blackstone’s senior managing directors and controlled by its founder, Stephen A. Schwarzman; and

(d) Clarus Ventures III, LLC (“Clarus GP LLC”), and Robert Liptak, Nicholas Simon, Nicholas Galakatos, Dennis Henner and Kurt Wheeler (together, the “Managing Directors”, and collectively with Clarus GP LLC, the “Clarus Persons”). The Managing Directors are the members of Clarus GP LLC. Clarus GP LLC is the former general partner of Clarus GP.

The persons named in this Item 2 are referred to individually herein as a “Reporting Person” and collectively as the “Reporting Persons.” The Reporting Persons have entered into a Joint Filing Agreement, dated as of the date hereof, a copy of which is filed with this Schedule 13D as Exhibit 99.1 (which is incorporated herein by reference), pursuant to which the Reporting Persons have agreed to file this statement jointly in accordance with the provisions of Rule 13d-1(k) under the Act.

The address of the principal business office of the Fund, Clarus GP and each of the Clarus Persons is Clarus Ventures, 101 Main Street, Suite 1210, Cambridge, MA 02142. The address of the principal business office of each of the Control Entities and Mr. Schwarzman is c/o The Blackstone Group L.P., 345 Park Avenue, New York, NY 10154.

The principal business of the Fund is to invest in and assist early-stage and/or growth-oriented businesses in healthcare and life sciences. The principal business of Clarus GP is to act as the sole general partner of the Fund. The principal business of Clarus GP LLC is to act as a general partner of investment vehicles. The principal business of each of the Managing Directors is serving as an employee of Blackstone or one of its affiliates with similar businesses.

The principal business of Blackstone Clarus III L.L.C. is performing the functions of, and serving as, the general partner of Clarus GP. The principal business of Blackstone Holdings II L.P. is performing the functions of, and serving as, a managing member (or similar position) of and member or equity holder in Blackstone Clarus III L.L.C. and other affiliated Blackstone entities. The principal business of Blackstone Holdings I/II GP Inc. is performing the functions of, and serving as, the general partner (or similar position) of Blackstone Holdings II L.P. and other affiliated Blackstone entities. The principal business of The Blackstone Group L.P. is performing the functions of, and serving as, the controlling shareholder of Blackstone Holdings I/II GP, Inc., and in a similar capacity for other affiliated Blackstone entities. The principal business of Blackstone Group Management L.L.C. is performing the functions of, and serving as, the general partner of The Blackstone Group L.P. The principal occupation of Mr. Schwarzman is serving as an executive of Blackstone Group Management L.L.C.

During the five years prior to the date hereof, none of the Reporting Persons has been convicted in a criminal proceeding or has been a party to a civil proceeding ending in a judgment, decree or final order enjoining future violations of, or prohibiting or mandating activities subject to, federal or state securities laws or finding any violation with respect to such laws.

The Fund, Clarus GP, Blackstone Holdings II L.P. and The Blackstone Group L.P. are limited partnerships organized under the laws of the State of Delaware. Blackstone Clarus III L.L.C., Blackstone Group Management L.L.C. and Clarus GP LLC are limited liability companies organized under the laws of the State of Delaware. Blackstone Holdings I/II GP Inc. is a corporation organized under the laws of the State of Delaware. Each of Mr. Schwarzman and the Managing Directors is a United States citizen.

On January 4, 2019, Blackstone integrated the Clarus business into Blackstone. As a result of the Clarus Integration, the Control Entities and Mr. Schwarzman may be deemed to have become indirect beneficial owners of the shares of Common Stock beneficially owned by the Fund (the “Clarus Shares”).

Clarus GP is the sole general partner of the Fund. Blackstone Clarus III L.L.C. is the sole general partner of Clarus GP. The sole member of Blackstone Clarus III L.L.C. is Blackstone Holdings II L.P. The sole general partner of Blackstone Holdings II L.P. is Blackstone Holdings I/II GP Inc. The controlling shareholder of Blackstone Holdings I/II GP Inc. is The Blackstone Group L.P. The sole general partner of The Blackstone Group L.P. is Blackstone Group Management L.L.C. Blackstone Group Management L.L.C. is wholly-owned by Blackstone’s senior managing directors and controlled by its founder, Stephen A. Schwarzman.

Each such Reporting Person may be deemed to beneficially own the Common Stock beneficially owned by the Fund directly or indirectly controlled by it or him, but neither the filing of this Schedule 13D nor any of its contents shall be deemed to constitute an admission that any Reporting Person (other than the Fund) is the beneficial owner of the Common Stock referred to herein for purposes of Section 13(d) of the Securities Exchange Act of 1934, as amended (the “Exchange Act”), or for any other purpose, and each of the Reporting Persons expressly disclaims beneficial ownership of such Common Stock.

The percentage of outstanding Common Stock of the Issuer which may be deemed to be beneficially owned by each Reporting Person is set forth on Line 13 of such Reporting Person’s cover sheet and was calculated based on the 29,038,308 shares of Common Stock reported to be outstanding as of November 5, 2018 in the Issuer’s Form 10-Q filed with the Securities and Exchange Commission on November 14, 2018.

Pursuant to Rule 13d-1(k) promulgated under the Securities Exchange Act of 1934, as amended, the Reporting Persons have entered into an agreement with respect to the joint filing of this statement, and any amendment or amendments thereto.

SIGNATURE

After reasonable inquiry and to the best of my knowledge and belief, I certify that the information set forth in this statement is true, complete and correct.

Date: January 10, 2019

CLARUS LIFESCIENCES III, L.P.

By: Clarus Ventures III GP, L.P., its general partner

By: Blackstone Clarus III L.L.C., its general partner

By: Blackstone Holdings II L.P., its managing member

By: Blackstone Holdings I/II GP Inc., its general partner

CLARUS VENTURES III GP, L.P.

By: Blackstone Clarus III L.L.C., its general partner

By: Blackstone Holdings II L.P., its managing member

By: Blackstone Holdings I/II GP Inc., its general partner

BLACKSTONE CLARUS III L.L.C.

By: Blackstone Holdings II L.P., its managing member

By: Blackstone Holdings I/II GP Inc., its general partner

BLACKSTONE HOLDINGS II L.P.

By: Blackstone Holdings I/II GP Inc., its general partner

BLACKSTONE HOLDINGS I/II GP INC.

[Gritstone Oncology, Inc. - Schedule 13D/A]

THE BLACKSTONE GROUP L.P.

By: Blackstone Group Management L.L.C., its general partner

BLACKSTONE GROUP MANAGEMENT L.L.C.

This Amendment No. 1 to Schedule 13D was executed by Robert Liptak on behalf of the individuals listed above pursuant to a Power of Attorney a copy of which is attached as Exhibit 99.2.

[Gritstone Oncology, Inc. - Schedule 13D/A]

EXHIBIT 99.1

AGREEMENT

Pursuant to Rule 13d-1(k)(1) under the Securities Exchange Act of 1934, the undersigned hereby agree that only one statement containing the information required by Schedule 13D need be filed with respect to the ownership by each of the undersigned of shares of Common Stock of Gritstone Oncology, Inc.

Date: January 10, 2019

CLARUS LIFESCIENCES III, L.P.

By: Clarus Ventures III GP, L.P., its general partner

By: Blackstone Clarus III L.L.C., its general partner

By: Blackstone Holdings II L.P., its managing member

By: Blackstone Holdings I/II GP Inc., its general partner

CLARUS VENTURES III GP, L.P.

By: Blackstone Clarus III L.L.C., its general partner

By: Blackstone Holdings II L.P., its managing member

By: Blackstone Holdings I/II GP Inc., its general partner

BLACKSTONE CLARUS III L.L.C.

By: Blackstone Holdings II L.P., its managing member

By: Blackstone Holdings I/II GP Inc., its general partner

BLACKSTONE HOLDINGS II L.P.

By: Blackstone Holdings I/II GP Inc., its general partner

BLACKSTONE HOLDINGS I/II GP INC.

[Gritstone Oncology, Inc. - Joint Filing Agreement]

THE BLACKSTONE GROUP L.P.

By: Blackstone Group Management L.L.C., its general partner

BLACKSTONE GROUP MANAGEMENT L.L.C.

This Agreement was executed by Robert Liptak on behalf of the individuals listed above pursuant to a Power of Attorney a copy of which is attached as Exhibit 99.2.

[Gritstone Oncology, Inc. - Joint Filing Agreement]

EXHIBIT 99.2

POWER OF ATTORNEY

KNOW ALL BY THESE PRESENTS, that each person whose signature appears below hereby constitutes and appoints Robert Liptak with full power to act singly, his true and lawful attorney-in-fact, with full power of substitution, to: (i) sign any and all instruments, certificates and documents that may be necessary, desirable or appropriate to be executed on behalf of himself as an individual or in his capacity as a general partner of any partnership or limited liability company, pursuant to Section 13 or 16 of the Securities Exchange Act of 1934, as amended, and any and all regulations promulgated thereunder, (ii) file the same (including any amendments thereto), with all exhibits thereto, and any other documents in connection therewith, with the Securities and Exchange Commission, and any stock exchange or similar authority and (iii) take any other action of any type whatsoever in connection with the foregoing which, in the opinion of such attorney-in-fact, may be of benefit to, in the best interest of, or legally required by, the undersigned, it being understood that the documents executed by such attorney-in-fact on behalf of the undersigned pursuant to this power of attorney shall be in such form and shall contain such terms and conditions as such attorney-in-fact may approve in such attorney-in-fact’s discretion, granting unto said attorney-in-fact full power and authority to do and perform each and every act and thing necessary, desirable or appropriate.

Each of the undersigned hereby grant to the attorney-in-fact full power and authority to do and perform any and every act and thing whatsoever requisite, necessary or proper to be done in the exercise of any of the rights and powers herein granted, as fully to all intents and purposes as the undersigned might or could do if personally present with full power of substitution or revocation, hereby ratifying and confirming all that such attorney-in-fact or such attorney-in-fact’s substitute or substitutes, shall lawfully do or cause to be done by virtue of this power of attorney and the rights and powers herein granted.

This power of attorney shall remain in full force and effect until revoked by the undersigned in a signed writing delivered to the attorney-in-fact.

IN WITNESS WHEREOF, this Power of Attorney has been signed as of the 9th day of February, 2017.