Gmod is an extremely popular Source Mod. I have personally only played the game for an hour or so, mainly to get an idea of how its used. I only bought the game to test Wall Worm in it and answer the large amount of questions GMod+WW users have. Gmod is a little bit different than the other mods.
Since most of my focus is on general Source, I'm placing this board under the control of func_Mathias who is a longtime WW/Gmod user. He can properly help people solve Gmod-releated issues like setup, compiling, ragdolls, etc.
Here are a couple general-purpose WW+Gmod resources:
Latest WallWorm and GMod documentation plus a dummy GameConfig.txt download (http://dev.wallworm.com/document/129/gmod_and_wall_worm.html).
http://www.youtube.com/watch?v=5LKCZl5omDU