"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'!"