The Gaster Followers say that one of his experiments… “shattered him across time and space”. And entry N° 17 says something like “THE SHADOWS CUTTING DEEPER”…
Maybe he discovered a Dark Fountain, or Dark Worlds, and got trapped here. We know Undertale has a lot more magic and stuff in it, so we don’t know how those work here. Maybe he created a temporary passage to enter one and it closed, trapping him (“HIM” hehehe) inside one.

How you think a CORE Dark World is?
Probably like the end of the Deltatraveler hardmode, just grayscale CORE.
What is . . .
” D e l t a t r a v e l e r “ ?
Welcome to the best deltarune fangame, man(?)
Are you new to theories or something like that…?
Because I’m pretty sure just about everyone has come to agree upon this…
Lmao nobody has ever talked in detail about this???
Much less agree???
It’s agreed gaster has SOMETHING to do with dark fountains in a vague unknown manner, if that’s what you mean?
I’ve seen so many people all saying this to the point I think it's a given.
But, most believe The Voice (Deltarune) to be a fragment of Gaster.
So combined with The Voice/Gaster talking about Deltarune (and the Prophecy, I’m pretty sure)…
The most logical conclusion of course, but I know this isn't a new discovery.
If the conversation goes to “who is Gaster” instead of the title, i’ll hunt everyone here one by one untill i’ve killed everyone and their Family

Everyone knows Gaster is only an Easter Egg and when he was created, Toby Fox wanted him to NOT play a major role in any game.
That is why he don’t has a confirmed appearance.
Or he's something Toby wanted you to find and solve. You know, like part of the plot of Ready Player One.
/necropostingagainiscoolnowig
Have you not read RP1?
Dude if you haven’t, you absolutely need to