CUSIP No. 372427104 Page 13 of 14

Exhibit A

AGREEMENT REGARDING THE JOINT FILING OF

SCHEDULE 13D

The undersigned, being duly authorized thereunto, hereby execute this agreement as an exhibit to this Schedule 13D to evidence the agreement of the below-named parties, in accordance with the rules promulgated pursuant to the Securities Exchange Act of 1934, to file this Schedule jointly on behalf of each such party.

Date: May 14, 2015

POLARIS VENTURE PARTNERS V, L.P.
By: Polaris Venture Management Co. V, L.L.C.
By:	*
	Managing Member
POLARIS VENTURE PARTNERS ENTREPRENEURS’ FUND V, L.P.
By:	Polaris Venture Management Co. V, L.L.C.
By:	*
	Managing Member
POLARIS VENTURE PARTNERS FOUNDERS’ FUND V, L.P.
By: Polaris Venture Management Co. V, L.L.C.
By:	*
	Managing Member

CUSIP No. 372427104 Page 14 of 14

POLARIS VENTURE PARTNERS SPECIAL FOUNDERS’ FUND V, L.P.
By: Polaris Venture Management Co. V, L.L.C.
By:	*
	Managing Member
POLARIS VENTURE MANAGEMENT CO. V, L.L.C.
By:	*
	Managing Member
JONATHAN A. FLINT
By:	/s/ Jonathan A. Flint
	Jonathan A. Flint
TERRANCE G. MCGUIRE
By:	/s/ Terrance G. McGuire
	Terrance G. McGuire
KEVIN BITTERMAN
By:	/s/ Kevin Bitterman
	Kevin Bitterman

*By:	/s/ John Gannon
Name:	John Gannon
	Attorney-in-Fact

[This Schedule 13D was executed pursuant to a Power of Attorney. Note that copies of the applicable Powers of Attorney are already on file with the appropriate agencies.]