and since the 가시나 it self is not a polite word at all ,normally it goes with 반말 but you can also say 저 가시나 누구에요 ( who is that girl?(with 존댓말) )

가시나 or 가스나 is a dialect in southern part of Korea where like Busan I guess( not 100% sure))and there is no difference between them. It’s not formal way to call a woman and mostly used when people call young girl in a bit rude way or maybe sometimes friendly way? It’s also similar like 기집애 or 계집애 which also mean young girl..Sometimes it can sounds a bit offensive to people depends on situation

you can use it like : 저 가시나는 누군데? (Who’s that girl?)