In my personal lore of Underswap, Gaster is swapped with River Person and River Person was the former "Royal Marine Biologist" who worked with Doctor Undyne until he died in a horrible laboratory accident involving an aquarium and liquid determination. River Person, aka the Royal Marine Biologist invented and built the Core along with Undyne and a forgotten group of scientists or "followers". 

As for Gaster, he's the River Person! His personality is a lot different, though. Very silly, bright with energy, pure of heart and kind. He makes money off of being a river transportation service for other monsters in the underground. He doesn't charge fare, but instead gets paid by the Royal Administration to do his work. He knows almost every river in the underground. He lives in a small Snowdin cabin with Papyrus and Sans.

Also, for those of you who are wondering, yes, these remixes are made by me. They are created in Logic Pro X. I rushed this one a bit during the end because it took me a bit too long but aside from that, I hope you all enjoy this remix of mine!

I found the picture from this source;
https://twitter.com/nttdj

Underswap - Core (By Grassinio)

https://www.tiktok.com/@grassinio_official
https://www.youtube.com/channel/UCbspqNQ9pZKGo32leoyBDfQ
https://twitter.com/greatgrassinio
https://www.twitch.tv/grassinio
Hello! I'm an artist from Saskatchewan, Canada and I specialize in video game remixes, but I also do some original stuff as well. I hope that you enjoy my content! Using my remixes is totally cool, just be sure to credit me.