It is an addon that you buy from steam to use with other games to give them more features.
Gary's mod is a Steam exclusive
Steam
You can buy the game off of Steam. You require a Source SDK game to run Garry's Mod, but Steam sells these games.
Buy Garry's mod 10 off of steam.
There is, 10$ Steam
There is no "Disk version" for garry's mod, you have to buy it from Steam
Assuming you're using Windows: C:/Program Files/Steam/SteamApps/{username}/Garry's mod If you want all the real G-mod files: C:/Program Files/Steam/SteamApps/{username}/Garry's mod/Garry's mod
If you use steam, right MB on the picture in steam and then delete.
All steam games update themselves automatically.
It sounds like you are trying to pirate Garry's Mod. Garry's Mod is only available through Steam, which requires the internet. There is no legal way to get it otherwise.
10$ through Steam
Just buy it on steam