Vanilla, how have you played Garrys Mod? Have you tried prop hunt, that can be pfun. I haven't tried any other GM mode yet, but at least Prop hunt works with a minimum of 4 players imo, so we just need to find a couple more.
As free alternatives for multiplayer, we tried Boardgame Online per FennecFyre's suggestion the day before yesterday. It was a, hmm, wacky "be-first-to-cross-finish-line" type of browser game with a jukebox, item inventory and random minigames. It started slow, and might not work with massive amount of people but it was pretty fun in the end.
FF also mentioned Town of Salem (a browser-based version of the "Werewolf"/"Mafia" game) and Paul mentioned Hedgewars, a browser version of Worms.