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”