The song or the wing ding wahoo man himself? I will assume the latter.
Generally speaking the things you have to know about my Current (this is a key operating word here because I change my mind on funny G man all the time) opinion on Gaster amounts to:
-I don't know a good word to fully explain this, but despite the likely stuff with Onion, I actually don't think Gaster is Inherently a fully antagonistic force, and I think Gaster-as-gamedev/game modder/rewrite maker both work in interesting ways with the parallels he has to Deltarune Gerson as an author who was swallowed by their creations. Deltarune, despite what many think, is an RPG that is /heavily/ informed by well... other RPGs.
Even within games which don't fall into the 'fight a villain who claims to be a god' format, there are enough instances of meeting a character who is in some way related to creation of a world that I think people assuming funny dings man wouldn't fit into Deltarune need to come back after they've played more rpgs.
The kitties are dual-purpose in the narrative : they're bugfixers/QA tester-adjacent (I think is what they normally are if you're not doing Egg stuff), but the fact they are good at finding 'protein' and are also described as pursuing a target until they get bored all feels also like they're granular lore fans who dig super deep into code to find stuff. Lorehead who also QAs is a frightening concept but we proceed.
I think the Big Thing that is most important to talking about Gaster is the 'MY DELTARUNE' line as it directly related to how The Prophecy is retold multiple times even within Deltarune itself!
If you believe he's the evil joker or whatever, Gaster saying 'My Deltarune' can have the implication that His View of what the end of The Prophecy should be is the only one that matters, or at the very least the way Deltarune as the main route plays out 'should' end a certain way. It's 'his'. His version of the story.
Note how I'm not saying 'the 'one ending' of Deltarune is the Original Intended Prophecy's ending'. This is because I think Gaster's Creation is less 'the entirety of Deltarune's world' so much as 'This Specific Interpretation of Deltarune's world'.
He's not the original world/setting's dev... he's a demiurge, and he's a modder at that... [gaster voice] Here's my deltaswap, btw
I'm going to toss the save file copying stuff in here, but I will also say that 'Deltarune's setting is a copy/reinterpretation of something before timelinewise' is kind of given evidence via Forgotten Man stuff, but I also think if we take 'copies are monochrome' at face value...
Gaster has taken his time to 'color it in', metaphorically. He likes the characters. He describes Kris, Noelle, and Susie as being 'wonderful'. :3
With this in mind, I for one personally would kind of argue AGAINST 'evil Gaster who only cares about his interpretation of the story' due to just how excited the guy gets when we do stuff like try and beat The Knight while missing the mantle, get no achievements in the playstation version, etc, but I do think that if Gaster expects anyone within the narrative to alter the prophecy, he's expecting it to happen as A Retelling After The Final Tragedy comes to pass, and not to be a divergence from the getgo (but well. He's excited to see it if it were to happen!) . Lord of The Hammer Cuts off in its final stages in a way that I think almost implies a 'repeated attempts to make the prophecy something kinder that have failed, and likely failed around whatever the sixth 'chapter' is in each thing' thing.
I also generally operate under the premise that as a result of this and the layers of stuff going on that theories that over-emphasize 'escapism bad' plotpoints don't work super well because of all of this; from the start, I think that the Light World being 'more real' than the Dark Worlds is a false assumption to begin with, to the point that I think the 'no Lightner merch' rule is going to get lifted at some point after the game finishes to Make A Point.
(I also think there /is/ supposed to be some weirdness about how dark worlds operate a la the like, meta layer of umineko, which I'm almost certain Toby Fox has played with how Aqua is written as well as the continued motifs of Ralsei furniture stuff, etc etc)
Also, apologies if what I just said is a bunch of words that barely mean anything, I have problems being coherent re: Gaster thoughts because I DO NOT actually have a fully good guess about what the end of Deltarune is going to be like.
