“Social justice warrior” and “SJW” have a history of usage that precedes #GG, and there are specific people and groups that were referred to by these terms.
Generally, “social justice warrior” has been a semi-derogatory label for advocates of identity politics, in the context of social media. As I pointed out in the Sup Marxists? thread, identity politics is a branch of political activism dating back roughly to the late 70s and 80s, and one frequently at odds with more “traditional” (for lack of a better word) branches of the radical left, specifically in that they do not assume solidarity among the oppressed as a fundamental principle and regard calls for unity with suspicion.
The older uses of “social justice warrior” that I’ve seen were actually mostly radical leftists criticizing them, and this continues to the present. That’s mostly in terms of critiques of strategy, theory, and so on, though there are some personal antagonisms involved, as you might imagine. It’s important to note that SJWs are principled opponents of sexism and racism, but I don’t think it’s accurate to use SJW as a general term for opponents of sexism, racism, etc.
There’s a small group of specific people, political activists active on Twitter and other social media, who are most often referred to as SJWs; I just recently found out that they’re all contributors to a magazine, Model View Culture.
More recently, as far as I can tell, this group of people have been targetted by right-wing trolls from 4chan and 8chan, in several “campaigns” that preceded #GG.
As far as I can tell, none of the primary targets of #GG – Quinn, Sarkeesian, Alexander, Wu, and so on – had any association with this particular group, and I’m not sure they’re even entirely aware of the pre-existing arguments. My sense is that the right-wing trolls pretty much bundled all their opponents together in the sort of semi-conspiracy theory we’ve been criticizing.