I had played gmod9 for a long time and bought gmod10 as soon as it came out lol. gmod is for hl2, but only requires you own hl2. But, to take full advantage of it, you also need dod:s and cs:s. however, you still got a lot with just hl2. Gmod allows you to build with objects found in hl2. Gmod it's self has a lot of stuff to build with, think its 500 things (a lot when you actully see) and it has mods for the mod (lol) that increase that a lot. test out gmod 9 first, as it is free, before anything else. But I highly reccomend buying gmod10, as its a small amount of money, just 10 bucks.
All times are (GMT -08:00) Pacific Time (US & Canada). Current time is 2:41:05 AM