Garry's Mod is a total conversion of Half-Life 2, making it into a sandbox game that can use content from Half-Life 2 and other Source Engine games. If you want to get this game, you will have to own at least one Source Engine based game (excluding Portal 2, Left 4 Dead/L4D2, or Alien Swarm.) Either way, I'd really recommend this and the one game you choose to get it. I'd recommend buying the Counter-Strike Source+Gmod or Team Fortress 2+Gmod bundle on Steam. But most custom maps require Counter-Strike Source, which makes buying the CSS+Gmod bundle more viable than the TF2+Gmod bundle. But the best option is to buy the Orange Box (Which includes TF2, Half-Life 2 and its Episodes, Portal, and Half-Life 2: Deathmatch), Counter-Strike Source, and then Gmod.
Also, I am running an RP server that is password protected, and if you want to get in, pm me and then I'll give you the password and ip.
phew, that was so much explaining. I wouldn't be surprised if you are confused about all of this.
-Aco