Saying "anti-GGers are a group" makes about as much sense as "people who like the colour blue are a group". They have similar traits and they might communicate with each other, but they don't have a real affiliation and they don't grant the whole group any more visibility just by speaking. The only way they can be classified in a group is if you yourself label them for convenience.
When you tag your tweets with #GamerGate, you're creating visibility to the group as a whole. And, as Twitter analysis has repeatedly shown, the vast majority of #GamerGate activity is concerned with its targets and defending itself rather than any actual ethical issues.
Now, #StopGamerGate2014 can be called a group. It's more upfront in its purpose, however. I haven't seen any complaints about the group aside from expected #GamerGate defensiveness.
As for the horrible Muslim argument that keeps coming up. Notice how many radical Muslim organizations have their own names. Most western Muslims don't want to be associated with them, actively denounce them, and don't use the names they use. Sure, they may all call themselves "Muslim", but there are denominations within that anyway. Denominations with their own names that they use to identify themselves. Muslim is an umbrella term with or without the radical element.
I think most people agree that moderate members of #GamerGate, the ones concerned about ethical issues and things of the sort, really just need to switch their label. They need to distance themselves from the soured one. They need to make it clear that they aren't the bad guys, that they're denouncing them, that they don't want to support them at all. As of right now, #GamerGate isn't even an umbrella term, everyone who supports it is supporting the whole, and the brunt of the whole is absolutely terrible.