Which game to install first?

Someone got me GMod and Counter Strike : Source and I need to know which game to install first and how to do it.

it doesn’t matter in which order you install them. =3

That’s what i call a dilemma!

I’m happy, that i don’t suffer such horrible and hard choices.

Alas, poor xVENUSx, the decision belongs to you.

It’s like run or shoot…

BOTH!

You don’t even need to install CSS unless your going to play it, if Gmod needs the CSS files it automatically downloads them since you have it on your steam account