Definitely. The movie draws elements and concepts from related comics to create its unique narrative. However, it also makes some creative adaptations and expansions to suit the cinematic format.
Multiverse of Madness isn't directly based on any specific comics. It takes inspiration from various elements of the Marvel multiverse concept found in multiple comic storylines.
There are fanfictions where Gaster is reunited with the other characters in unexpected ways. For example, a story could have him reappear during a crucial moment in the Underground's history and use his knowledge to change the fate of everyone. It could be really exciting as it might bring new plot twists and character developments.
The war between Gaster and Grillby in the fanfiction might have been sparked by a personal grudge or a competition for influence. Maybe there was a hidden secret that drove them apart and into conflict.
The 'Multiverse of Madness' is a wild and exciting story. It involves Doctor Strange exploring different universes. There are various versions of characters he meets. In this story, Strange has to deal with powerful threats and unexpected allies. The multiverse concept brings a lot of chaos and wonder.
Some characters might be in awe. For instance, Thor could be excited to see different versions of himself and his friends. He might see a version of himself that is even more powerful or a version of Loki that is actually good, and it would be a shock but also a source of amusement for him.
You can start by looking on popular fanfiction websites such as Archive of Our Own. Just search for 'Undertale Gaster' and you'll get a list of related fanfictions. Sort them by popularity or rating to find the ones that are well - liked.
Obviously, Gaster himself is likely to be a main character. His mysterious nature makes him a central focus as readers are eager to learn more about him. Another potential main character could be Frisk. Their ability to reset and influence the game's world could be very interesting when combined with Gaster's enigma. They could either be allies or have some sort of conflict due to their different understandings of the world.