>>534087981
It's more akin to a fangame, which makes sense considering the whole themes of escapism and creating fiction and reinterpreting stories.
Gaster saw the original timeline of Deltarune play out and he became invested in the world but he didn't like what happened at "the end"
So he endeavoured to create a new ending, he may have made some big change(s) somewhere that we aren't entirely privy to, but he doesn't have enough power to personally physically interact with the world of Deltarune as much as he'd like, he needs our help to complete it.