CS:S and GMod?

get cs

you need CSS for it to work or any valve game but CSS is recommended since some sweps require CSS for them to work.

if u plan on playing garry's mod i recomend getting as many source games as possible

Yes I would recommend you to get it along with CS:S.

I'd get it along with CS:S , Many gamemodes use the textures etc etc.

Buy the GMOD and CS:S pack. Most game modes use CS:S textures, and trust me, you don't want to be playing without textures. You can, but little purple/black squares replace the missing textures