If this is on minecraft then you go to %appdata% and go to minecraft inside minecraft go to worlds folder get garrys mod and paste it in that folder. the best download is www.minecraft.com/garrysMoD/Download114232
Off of Steam (10$) (http://store.steampowered.com/)
You Haveto Buy It. On Steam Its 6.99
There is no "Disk version" for garry's mod, you have to buy it from Steam
Steam
Buy Garry's mod 10 off of steam.
There is, 10$ Steam
Yes because you can't play Garry's mod without steam.
10$ through Steam
Garrys Mod 10 Is available on Steam, Which can be found at SteamPowered.com You First need to get a Source Game, which can also be found on Steam, that is including Counter Strike: Source. All and all it should cost about 21 US Dollars.
Garry's Mod has now updated and includes all features of 13 Beta
Go to garrysmod.org and download various addons. These after download are usually extracted to the [Steam Install Folder]/steamapps/[Steam User Name]/garrysmod/garrysmod/addons folder. But these addons have installation instructions most of the time.
You Do Not Get Garrys Mod Serials, You Buy it Off Of Steam Which can be found at steampowered.com its usually about 21 United States Dollars