Garry's Mod

I have been playing for Counter Strike for many years. I actually started to play this game since 1999 when Half Life 1 came out. Then, counter-strike beta came out few month later. I was a high school student and this game is only stress-reliver for me. I played a lot I mean A LOT of FPS game since then, including ID's Doom series, Halo, etc and as I get older, recently I stop to play game anymore. However, from 2 years ago, I used to play Crysis because the jaw dropping graphics and the physics of game are absolutely amazing.
On thursday, Professor gave us a brief introduction on Garry's Mod. I was wondering if I have that game in my Steam account and checked as soon as I got back home. I HAVE IT. I think I bought this game with Valve's package or I can't remember exactly but since I was a Half-Life 2 player, I was able to play easily. I'll try to make my city as one of student demonstrated during the classs on thursday.