A sexual act where two or more gay men engage in penetrative sex with their assholes. This is usually accompanied by a copious amount of lube. According to the Geneva Convention it would not be considered gay butt sex but rather just butt sex if one of the parties involved utter the magic words "no homo".
Charles: Yo I had Gay Butt Sexyesterday. It felt amazing!
Bao Dao: Yeah I know I did it with you!