In any gamemode I've played, the player models always use HL2:DM animations, even when the games involve deathmatching or use CSS models. I could be a little bit too general, but I think it's agreed that the HL2:DM animations look crappy, awkward, and stupid and are in general inferior to CS:S animations. We already know that animations from other games can be incorporated into GMOD, such as this TF2 gamemode for Garry's mod.

http://www.youtube.com/watch?v=ztYN-FEOLSY&feature=channel_video_title

We also know that CS:S animations work in Garry's mod if you have ever used the model manipulator and replaced an NPC with a CS:S playermodel. Is it due to just laziness that most people don't want to use CS:S models? I'm pretty sure an easy workaround for the newton launcher, RPG, and crowbar in TTT if CS:S animations are ever incorporated is to swap the CS:S model rigged to the skeleton in place with the CS:S model rigged for HL2:DM animations.