1) A person who likes the Show Stargate - SG1 and/or particpates in Stargate Fandom

There has been some discussion as to where the term orginated, the general opinion is that it orginated in on the newsgroup alt.tv.stargate-sg1 In a discussion as to what stargate Fans are called. The term was fully cemented into Stargate fandom when the forum Gaters.net was set up(This forum as a branch-off of SG1.net when it closed down, SG1.net Was founded by the same person (Sean Fitzgibbons) who founded alt.tv.stargate-sg1.
The term also reached *Offical* status when Brad Wright(Executive Producer) said fans call themselves "Gaters", on one of the DVDs

I fluffy dog like creature that can’t seem to play hide and seek properly without a tragic accident.

A fan (enthusiast) of the Stargate franchise (Stargate movie, Stargate SG1, Stargate Atlantis, Stargate Worlds, etc)

1a. a person who is a fan of Stargate, either the movie, the TV show SG-1, Stargate:Atlantis, or all three. (or just two out of the three.)

1b. a member of a secret bulletin board hidden in the depths of the internet. This bboard...well, lets just say...

T3h poster of the GAT, looked up to as a g0d