Start by downloading blender unless you have some other good 3d program and start practising texture painting, you can also use a program like substance painter. Texturing can be quite hard, but if you want to do it on any model (from a gmod addon) here's some steps, you may need to search some stuff up
1. download the gmod addon
2. extract it from gma to some folders
3. decompile the .mdl (I use crowbar)
4. Browse to the materials, extract them to vtfedit and save them as .jpg or something
5. Import the models into blender, and paste the materials on them
6. Edit the materials to your own likings, make sure the materials keep the same name as the old ones
7. Save the materials, open them in vtfedit and replace the old vtfs in the material folder
8. Compile the model
9. Search up some tutorial on youtube how to make a gmod addon out of it
This is is an extremely basic guide and you'll probably run into some errors sometimes but it should help you on your way.