It's a steam bug, happens with most source games atm, just restart steam, should work.
I've restarted my steam and computer.. still doesn't work :p
does it say "Registry Error", if it's a message box?
Anyway, exit steam, mosey on over to your steam install folder and delete clientregistry.blob, start up steam.
If that doesn't work, try other games *HL2/TF2 - Source games* and see if they work, if they do then it's a problem with your Garry's Mod.
If it IS a problem with your Gmod, Verify integrity of game cache, reinstall, w/e.