On my 2 P3 450 with Matrox G400 AGP, gmc reports a segfault at startup, and keeps segfaulting as it respawns. After removing gmc from the automatic startup menu, I started it by hand and it reports bus error. I have a core dump, but no symbols so I can't get anything useful from the back trace.
Can you log in as a different user and see if gmc still crashes (or move your .gnome and .mc out of the way temporarily, if you can't make another user.)
Logging in as root worked, i.e. no gmc fibrillation. I then logged into my wife's account; it still failed with gmc respawning. Exited to a console, removed all core files, started X again, can't reproduce the problem. Color me covered with egg. Sorry Jon. I'll keep an eye peeled for more problems, but it appears to be some other instability.