"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'!"