If someone were to ask me what "GamerGate" is, I would say "A set of events which consist of a bunch of press scandals, a proxy war between SA and imageboard culture, a proxy war between Youtubers and traditional games journalism, the pushing of specific ideologies by the press for corporate profit, resistance to the creation of an ideological hegemony within geek culture and gaming, widespread censorship and the attempts to route around it, the creation of a hashtag with the same name, and an emergent protest against the state of the gaming industry that resulted in a consumer revolt."
There's more to it than just that too, but that is as succinct as I can make it.
To me, a "GamerGater" is:
"A combination of a thought terminating cliche and a political dog whistle created by the gaming press, in cooperation with Chris Kluwe and 'geek celebrities', in order to establish a label for elements of their out-group that they dislike. This was later adopted by some of the participants within the consumer revolt as a self-applied in-group identifier. Self-identification is the only criteria for being a gamergater, otherwise the individual participating in the revolt is an individual with their own goals and motivations, and should be treated as such."