What does it even mean to stand against Gamergate?
It can mean all sorts of things, I suggest you open up a Thread about it.
Are they standing against harassment?
Yes, they were standing against
some harassment, but though many people on both sides of said conflict would gladly have you believe it, any people who were conceptually against harassment and somehow got mixed into the (about entirely different things) GamerGate discussions were few and far between.
Why don't they just say that then?
They do, but since "harassment" talk is more about creating a meta-narative around the original contentious issue, it will only go so and so far in achieving the true goal. Harassment talk is about attacking the character of your dissenters, it's trying to win the "Debate" by subverting the "Debate".
The Video starts out bad and gets worse over time. They made poor arguments for a dumb proposal. Since they find themselves rightfully criticized and mocked for it, they have to appeal to emotion by representing the dissent as harassment and putting themselves in the Role of the Martyr, this in turns is done in hope's of galvanizing support without regard for the WHY because no amount of being wrong justifies Death/Rape Threats (and we've been ignoring the impotence of such threats for a few years now, so there's that).