Part of the reason is that the commercial clients have finally eclipsed gitx in power, ease-of-use, and prettiness.
Notably, SourceTree (what mainly use) and Tower (if the incomprehensible one-repo-at-a-time limit isn't a dealbreaker) now compare favorably to gitx in most ways.
So I don't recommend gitx anymore, but from what I hear Gitx (L) is a fairly active and popular fork:
Been using that for a few weeks, it's great. I previously used brotherbard's experimental branch from github and he told me about it.
I use GitX for committing and browsing and the command line for the rest. None of the GUIs have blown me away yet, but deep GH integration is a big plus for me so GH for Mac might join my workflow.
Notably, SourceTree (what mainly use) and Tower (if the incomprehensible one-repo-at-a-time limit isn't a dealbreaker) now compare favorably to gitx in most ways.
So I don't recommend gitx anymore, but from what I hear Gitx (L) is a fairly active and popular fork:
http://gitx.laullon.com