Not sure if I'm understanding this. From what I read in your post there's 3 different Gasters you mentioned. Undertale!Gaster who created the Deltarune verse, Light World!Gaster, and Dark World!Gaster? Which Gaster is the story focused on?
ok so as we know from gaster follower #2:
They say he shattered across time and space.
Ha Ha… how can I say so without fear?
I’m holding a piece of him right here.
and so heres how it goes.
before agf movie starts, there is only one universe. this is UNDERTALE PRIME, the original timeline/universe, where W.D. Gaster is the royal scientist. he falls into his creation and it shatters him into pieces across time space.
it also splits and reworks the timeline into a new universe, DELTARUNE PRIME. This is not exactly the deltarune we play in, this takes place in the past and goes under slightly different rules.
so keeping this as simple as I can, DELTARUNE PRIME is the original undertale universe split into two halves. its split into light/dark and reality/fantasy. this is because there is only ONE dark fountain, which was made by gaster through the experiment that caused all of this. this fountain creates their forms and connects both worlds, the people themselves are split between their hopes and dreams. they’re separate halves of each other.
this is the fountain that ralsei’s empty town is around, these people were the people he would of governed. the second fountain hasn’t been made yet, lancer and every other darkner doesn’t exist yet. darkners aren’t toys brought to life, they’re the imagination/dreams the lightners had of the toys brought to life. this fountain gives form to their literal dreams.
i know its like. so lore heavy and you wouldn’t believe how much time my and my bud had trying to figure out all this lore… its been crazy. its been such a headache and i’m still not even fully satisfied by this explanation, it doesn’t even fully fit my h/cs but! its the best i can do and works for the story. i wanna make it clear that all this heavy lore stuff is just the set up for the world, its not the main focus. i mean, its a big mystery to the characters and they try to figure this out, its a big thing in the story, but its not what really matters! its the characters, and the allegory/moral i want to write. but i hope this helped at least make the premise clear?