I don't use GMod, but yes, you can use Blender to make models for GMod. You can search around on blenderartists.org (and facepunch too, I'm sure) and you can find some tutorials. However, most tutorials are written with the idea that you are already familiar with Blender, scripts, textures, and how it all works together. Most tutorials do not go into any kind of detail.
I would learn the basics first; it will save you a lot of frustration. Seriously, starting out doing something complicated when you don't know a lot about it will drive you crazy.
Of course, if you do know the basics and can work with 3D programs fairly well, I hope the sites I mentioned earlier will help you out. Best of luck! 8-)