So i know a few maps have done this, where they would use a 3D model as the Entire map or just some of the map. Now how would someone even do that? I assume i would have to port the 3D model to Source Format, But then again I'm guessing the collision would be hard to get right depending on the model. Basically what I'm trying to do is take a model of a map i ported from another game and try to port it to Gmod as a Map. If it can be done please let me know^w^
It's perfectly possible, as you mentioned; collision accuracy is the main concern. You can circumvent this by disabling the model's collision and recreating the "map"'s structure out of clip or nodraw brushes. Clip brushes do not interact with physics objects. If your intent is to turn an entire map from another game into one model - you're very likely to suffer poor performance. If it's just one room that you intend to turn into a rats type map - yea that'll probably work fine.
also your lighting will be shit
 there's also something about source not liking models w concave or generally just complex collision meshes but I'm not personally familiar with that, feel free to read the wiki yourself