Well there is more to Garry's mod than just Garry's mod. You play fun game modes like Trouble in Terrorist Town (if you can deal with the kiddies) and a huge variety of other game modes to play with or without your friends, and that's just the multiplayer, on single player you can spawn dupes and mess round with them, everyday there are pages of new dupes!
I've played 377 hours of Garry's Mod with other people so it isn't that bad., and maybe you'll make some friends on Garry's Mod to play with anyway!