gaster's voice can be heard in spamton neo's theme 😳😳😳😳😳😳😳😳😳😳😳😳

I'm genuinely curious as to what plain of existence Gaster is on in Deltarune, it's clear he exists because of the first interaction as well as the Easter egg during that interaction in Chapter 1, but what is Gaster, is they a Monster? A Lightner? A Darkner?