He's a Gamergater because he has slapped #gamergate on the ends of their tweets and spoken in support of them. He openly supports that group.
Gamergate was originally the Quinnspiracy, a movement which existed for the sole purpose of harassing a woman, Zoe Quinn. That's the purpose for which the group was created, and harassment of women continues to be its main purpose.
If you go to Gamergate's spaces, you see shit like this. If he has communicated with other Gamergaters at all, he would have bumped into things like this.
The group also has a significant openly Neo-Nazi component, and courted groups like /pol/: