It is a long, stupid story. This youtube link does a decent job of going over. The drama between the individuals that sparked it is not important to anyone but them. What makes Gamergate significant is that it eventually turned into the red line beyond which even default lefty young people hit their limit with the SJW commissariat, and told them to bug off.
In the culture war, in that age group, it was a Battle of Midway moment. It may seem trivial to most of us on this website, but it was a serious moment in the culture war for Millenials and Generation Z. It was a cold hard rejection of dictated PC.
Here is the link. Language warning.
https://www.youtube.com/watch?v=STl7-_f4_eA