Rizz = charisma = good flirting. A rizzler is a person who flirts well.

Gyat = Girl Your Ass is Thick. And is also used as an exclamation upon seeing a thick ass and a term for a thick ass in general.

So rizzler gyat is a flirtatious girl with a thick ass.

Also of note “sticking out your gyat for the rizzler” is the song that blew up those words would mean “sticking out your butt for the charismatic guy”

Attractive person’s ass