The GPLv3 requires that you mark your modifications:

Conveying Modified Source Versions.
You may convey a work based on the Program, or the modifications to produce it from the Program, in the form of source code under the terms of section 4, provided that you also meet all of these conditions:

a) The work must carry prominent notices stating that you modified it, and giving a relevant date.
[…]

Not exactly requiring a git repo, but a git repo would make it easier to comply with this condition.