The first rumblings of this all were when a group of SJWs took negative criticism for the game Depression Quest on a site called wizardchan as harassment, and used the justification that the gamer nerd misogynists therein were scum and anti-women to attack gamers in general. The label has since shifted to those who tried to defend them.
It got amplified when The Fine Young Capitalists started an IndieGoGo campaign to support women in game development, and, when 4chan's /v/ decided that, as a measure of good faith, they would donate large amounts of money to the campaign to help counter accusations that they are anti-women. As a result, TFYC were suddenly attacked as anti-women, doxxed, and had their campaign temporarily taken down on altogether spurious charges.
The thing that caused GG to become a movement, the last straw that inflamed it, was a large number of articles written across two days from a number of gaming news sites proclaiming that 'gamers are dead' and that gamers are all misogynist. This continued to spread the narrative that GG was designed to attack women.
GG's primary focus is on journalistic ethics, but has had the constant need to fight off the argument 'but you hate women so anything you say is invalid'. So, GG makes an effort to support women and anti-bullying, as proof that they are not purely scum. It's rather horrifying to watch people try to ask the charities to stop accepting money from people who are donating because they are apparently lying.