Hard drive space disapears (n00b question)

Bought my kid Garrys Mod because he saws videos on uTube and thought it was cool. Anyway, his PC meets the recommended requirements posted on the Steam Store. He had about 3 gig free on his hard drive, so the 150 meg download was not so bad compared to other Steam games. Once he started it, it filled up all but 256 meg or so of his HDD. Does Garrys Mod download other game files? When I get him a larger drive, will the overhead stay about the same?

If he didn’t have half life 2 installed it had to download those files in addition to gmod files.

Launch GMod, dismount any games you don’t want mounted, and delete the gcf of said games. GMod downloads other source games that you have purchased in order to mount them, so disabling mounting them will remove the need to have them.

Thanks folks, I figgured it was something like that. He did have HL2 installed, but not a some of the other Source games he owns. We deleted them to save space. SO Garrys Mod just needs the GCF files available, not the game actually installed?

How does one dismount the games? If he does, won’ the resources become unavailable? Forgive the basic questions, I am a 50-something old school gamer, used to run and gun. Anything more than move, shoot and duck gets complicated for me.