The more I read about Gaster theories of him in Deltarune the more I am convinced he's a red herring and that's its either not gaster or if it is his ass is not playing that important of a narrative role but tbh i think the main reason I dont like the theory is it really just makes it feel like Deltarune can't stand on its own merit
I dont even like it when people say play Undertale before you play Deltarune because given how much Toby has insisted that despite there being connections between DR & UT they aren't the same, they won't be the same game and even going on record to say "I can't give you undertale again"
People who know nothing about Undertale who play Deltarune would never arrive at the conclusion of Gaster and Toby has played with expectations before he knows we want an answer to the Gaster mystery he has routinely trolled his audience before and it doesn't change the fact we only know of Gaster due to the fact we've had ten years to dissect this game & again undertale blew up in popularity
You can play vanilla undertale a hundred and never encounter anything gaster related, and dont get me wrong when you do get a gaster type encounter, its fun to headcanon, its fun to speculate and it has interesting world building implications past the main story but part of what made it fun was there was no answer
Gaster is at his coolest when he remains a mystery thus while I'm mad skeptical the person pulling the strings is Gaster I'd like to think that even if it is him he is either very minor in his role or remains a hidden figure we'll never know 100%
Hell, part of the reason that I am a little critical of "Sans is originally from Deltarune" despite that having far stronger evidence (because Sans is an active character in both UT & DR go figure even if its more minor in the latter) is because its using Deltarune to fill in the blanks of Undertale which i just dont care for
Sometimes things are meant to be unexplored and better a mystery