"GL" or "gl" in the manga, manhwa, manhua universe means
literally girl's love. Which is a romance genre when both of the main characters are females. Often interpreted as "
Yuri" for the Japanese community which means Lily. The fans of the genre are simply called
Yuri enthusiast/fan. Another variant for the opposite sex is bl, which you already know it, means boy's love (I'm not into that).
Me: "Hey, did you read the gl manga I recommended you?"
Yuri fan: "Oh, Kimi ni Tsumugu Bouhaku? Heck yeah dude, that manga sure be
vibin'!"