Short for "Good Luck"

gl stands for

Good Looks or Good Look
which is a way of thanking someone

Depending on which term you use it in it either means good looking or good luck

GL:
1.Girl Love
Basically a genre of comic

Like BL but with girls

2.Good Luck

Good luck or gay lord

means good looking or good looks

Gl is a genre that depicts homosexual relationships between women and focuses on emotional aspects of relationships.
Additionally, people use the word "yuri" but yuri focuses more on the sexual aspects(explicit content) between women.