Garry's Mod hard drive problem

Hello

I bought Garry’s Mod on steam, DL’d it and lauched the game and it started downloading a looooooot of materials. Remaining space on my HDD dropped from 16 Go to 600Mo
How can I avoid this problem ? Thanks a lot

(Totally unrealated) Did you buy CS:S or HL2/1? Also, Garrysmod should not take up that much space.