What if Gaster was split, into mystery man, the voice maybe some others like the goners or something
My guess: When Gaster experimented with timelines, he split himself into two forms, one in Undertale and one in Deltarune. I think for sure he would have split into Mystery Man and possibly The Voice.
He was split
Across space and time
There is both physical (space) and chronological (time) space between Undertale and Deltarune.
Also that would make it possible for gaster to do determination experiments similar to the ones in undertale resulting in Eram and FRIEND
I think that the Goners/Gaster followers had witnessed Gaster break through time and they had been physically and mentally affected.
I mean if I saw my boss get shattered I think I'd be scarred
If you take what the Gaster Followers say and put it together, it comes out like this:
Gaster had been working on an experiment in The CORE, but something had gone wrong and he fell into his experiment. Whatever he was working on shattered him into Mystery Man and The Voice in two different timelines.
What if the knight is just getting monsters and turning them into amalgamates
What if Gaster broke his own mind but kept his powers.
And is...
Sans
If you think about the similarities, the fonts, the "Gaster Blaster", Sans's ability to see timelines, and his odd workshop blueprints in unreadable symbols.
I think he worked with gaster because in entry seventeen he says "what do you two think?"
No, both Sans and Gaster seem like they prefer to work alone...
But it's in gaster's entry seventeen, he had to be with someone, he isn't talking to himself
Maybe sans likes to work alone because of that day?
Wait! Go back to what I had said, he split into Mystery Man and The Voice and Sans.
Yeah after he fell in he was together during entry seventeen
And then the part of him that was The Voice had left when "Photon Readings Negative
Technically possible
But he says two
I think it was Sans talking to Mystery Man and The Voice, but most likely MM and V were opposant mental sides of Gaster, so Sans made a Dark Fountain and separated them.
And then he regretted it causing him to not want to work with other people
I like that
Therefore, Gaster split into three, but V had gone into a Dark Fountain and Sans and MM never met again, because MM was mad at Sans (Who was the only one with the power to do it) for making the DF