According to https://github.com/git/git/commit/62c814b6b60f5b5605e34c9806d874fe1088d416 the git-remote-helpers works for Python3 with the help of 2to3, since version 1.8.2 Reproducible: Always
The problem isn't the remotes, it's that dev-python/pygtk is only available on py2.6/7. Modify the git ebuild for PYTHON_COMPAT=( python2_{6,7} python3_{2,3,4} ) and run repoman full. Get pygtk fixed first, then reopen this.