I checked out on Garrysmod.org and there are these missions, how do u play them?
Huh. What missions? Care to link?
I think he means the save files like “Kill Dr. Breen” and shit.
None of them are with your time.
But if you really care, it’s a matter of installing the shit like anything else, but it generally includes a save file. So drop that in the save folder.
They usually include a README file explaining the extremely simple process. They also all suck. Just go play HL2. 
Who even made first idiotic ‘Kill Dr. Breen mission’? It’s so damn annoying to see them being added everyday.
Like dArKnEsS_2 said, play HL2, or some other game, not those boring Breen missions.