Would it be possible to make the Ai from left 4 dead/2 or something similar to that for gmod?
What do you mean?
No, gmod would have to be ported to the engine that is used by L4D(2) in order for stuff like that to work.
More like, gmod can’t use the l4d maps’ noding system, I would think. But that isn’t to say you couldn’t have a script do it.
Gm_Navigation module can create node files for maps, but you’d also need an npc that could use the nodes.
So yes it is possible, but you’d have to code the npcs yourself.