What “Gyal” means
Gyal means "girl" or "young woman". "Mi gyal" can mean my girlfriend.
Examples
Di gyal pretty eeh.
That girl is pretty.
Da gyal deh, shi can sing!
That girl, she can sing!
How to use it
Everyday word, and very common in dancehall lyrics. Tone matters: between friends it's normal, but calling an older woman "gyal" is disrespectful.