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About Garry's Mod
Garry's mod, ironically, can have mods attached to it. Stargate is another one of Garry's mod's mods.
The Stargate mod lets players use technology available in the popular Stargate TV series, such as Stargates and sheild generators. It's very common for Stargate to appear on flatgrass "build only" servers where players work togeather to create worlds and portals between areas of the server.
The addon must be downloaded unofficially and extracted to your local game files from websites such as www.garrysmod.org. Here's a direct link to their Stargate downoad page: http://www.garrysmod.org/downloads/?tag=Stargate.