How would I make these into gmod models?

Well I have acquired a model of Darth Maul, and the files I have are:
cis_inf_darthmaul
cis_inf_darthmaul_cape
{Those were just texture files saved as png files)

But after that I have:
Darth Maul.DAE
Darth Maul.mtl
Darth Maul.obj

Is there a program I need to do this with, or is it impossible?

Try putting the files into a folder and extract that folder to garrysmod/garrysmod/models then with the same folder extract it to garrysmod/garrysmod/materials.
Best I can think of.