THE COMICS RESOURCE

[Rosencrantz]

Actually, while Source SDK is really good to have as a checker, you don't need it at all to port models into GMod~

You'll need at least Notepad++, MDLdecompiler, and a 3D program that can export .smd files (Blender is very good with the imports AND exports [plugins are available on the wiki] and I personally use it). GCFScape is a plus, since it can rip out the hidden .qc files from various Source games for you to edit.

For a tl;dr of what you need to do, you'll need to:
1) export your .smd file
2) write/edit an appropriate .qc file
3) have MDLdecompiler run the .qc and churn out a usable GMod model
4) package the files into the appropriate GMod directory with their appropriate texture .vtfs and .vmts in their required folders (defined in the .qc)
5) run GMod

It's not a terribly complex process, but I might have forgotten to mention a step in there (if someone would so kindly correct me C. The hardest work involves making sure you export the correct models (they need to include the skeleton rigging) and writing out a good .qc file.