What NPC's are you talking about? If you spawn resilience and zombies, the resilience will attack the zombies.
NPC's are programmed to do as that of HL2, you just have to spawn them with weapons...
Well in episode 2 when Alyx got close to a zombie sometimes Alyx would kick the zombie, or the zombie would attack and she would dodge. But if equipped with a shotgun she would smack the zombie.
The vortigaunt does this as well sometimes the vort will stomp on an antlion, or grab it and use the green energy beam thingy. And the other one I recall is the hunter and any human like citizen,kliener those types. The hunter would knock the human over and stomp on them or it can impale them.
GMod uses it's own branch of the Source engine, basically it's a mix because of mounting other source engine games. So in this sense, it's modified as it can handle more than 2 dynamic lamp sources and able to use content from other Source-driven games.
It has every feature, and at the moment GMod 13 beta has a newer branch. So there's improvement or losses and it's community built this time.
That sounds pretty awesome I can't wait till GMod 13 is out.