Yeah if you have Half-Life 2 and you don't have Steam already you should get it. You have to to play any mods anyway.
That's what I mean, I don't understand how I can play GMod and not CS Source. I get CS works OK sometimes but the other mods like the new Battle Grounds 2 and Plan of Attack... they lag waaaay too much for me to play. But Garry's Mod doesn't, for some reason.