Engine version?

What version of source does gmod run? ep2, l4d, portal 2?

It runs the latest TF2 engine. (Which is quite different from the EP2 engine.)

I thought it was the HL2:DM engine?

HL2:DM, CSS, TF2, GMod and some other games all run on the latest build of the source 2009 engine.

I thought it used some form of the Orange Box engine… Or is that what your referring to when you say “TF2 engine”…?

It’s the 2009 Source Engine. L4D runs on it’s own variation and that is why the maps do not work in Gmod.