Two opposing thoughts.
1. We can keep git-gui and git proper separate projects, move git-gui
documentation out of git to git-gui, and with clever Makefile trick
include and build git-gui related documentation conditionally only
when git-gui appears part of the git project tree (this anticipates a
future where git-gui is bound to git not with the subtree merge
strategy as we currently do, but as a submodule). 2. Just like the Linux kernel project, we can make each subsystem with
separate maintainers just different repositories of the same git
project with their own focus. We already do this for git-svn (which I
delegate to Eric Wong by pulling from him) and some parts of contrib/
tree; we have already been halfway there for gitweb/ (which I don't
regularly "pull from", but I mainly act as a patch monkey without
actively managing that part myself). I don't see why we cannot extend
that model to git-gui and gitk.