Some interpretations of Fell!Gaster (Non-canon)
A version of the scientist Gaster which is a more intense and aggressive than most versions of Gaster (or at least he is meant to be, some versions of UT Wingding actually do things far worse than what UF version of him do). Like most UF interpretations of any character, they lack compassion. His relation to Sans and Papyrus also vary.
The cruel but not evil:
A version of the scientist Gaster whom is cruel but has a level of sympathy for some other monsters. They may do terrible experiments that sometimes result in death but they have spared monsters before. (This is based off the Maxlad version as seen here.)
A version of Gaster who isn't a scientist. He is a general in Asgore's army instead. Despite being a fell creature, he isn't evil, he's just a jerk. This particular version has a hard time smiling, and has been a Prisoner of War. This specific version of Underfell Gaster has a fear of fireworks and spiders. (This is the Bunnymuse version of Underfell Gaster as seen in the GasterGang)
The Good Version:
In some interpretations he hated the motto of King Asgore and worked on ways to either bring the Kings children to life, or reinstate the kings sanity. He was pushed into the core by Asgore, and in some cases it was Papyrus.
- hes Gay