Online people will just assign you whatever you seem like. If you play games: boy. If you play games as a support character: girl. If you play Yasuo: boy. If you watch hentai: boy. No matter what people will make an assumption. You can correct them, but it isn't necessary. Especially in games. If people think you are a guy, then they don't act weird.
I never understood why playing games is boyish. Isn't it evident that people like to have fun? The most miserable adults I met all thought playing games is childish...
It happens a lot online, so there's no need to concern yourself. As Envy said, people online will assign whatever role they see suitable for you when it's not immediately obvious, based on taste or their perception of you—this is because people's minds are more gendered than they like to admit. At the same time, it's the 'role' they fill, and your actual gender doesn't matter most of the time.