What would it take (besides hexing) to do as the thread title says? Would it be as simple as adding the model to the playermodel list or would I have to decompile it and modify the qc or the model itself in order to make it work with Garry’s Mod’s player model systems? It’s the same skeleton, as far as I can tell, so I’m hoping it would be a fairly simple process. Of course, I’d get permission from the original authors as well.
You would most likely have to recompile the models and QC to make use of the animations for playermodels.
The Left 4 Dead 2 skeleton actually does differ from the Garry’s Mod skeleton, albeit not too heavily. It’s primarily the clavicle rigging as well as the upper spine in the thoracic and cervical regions and the thumbs which are backward. Other than that the differences aren’t very prominent and you should be fine, though as Ben said you’ll need to recompile the model as hexing won’t do much good.