SapphireRayNoel
Gaster knew how Sans' condition would put him in a disadvantage when it came to the second war that it seemed the humans would soon start. In an abandoned lab on the surface he takes Sans and Papyrus there, Sans being asleep.
He put the small skeleton into a machine and the machine wrapped him in a crystal. Leaving him asleep for years, not aging.
When Sans wakes up he can't find his father or his brother. Frantically looking around he finds Papyrus one day by running into him on accident.
Naruto is neglected and he runs away from home. He finds a mountain and sees the hole out of curiosity he jumps down the hole knowing he won't die thanks to Kurama.
He meets the wonderful monsters and decides to free them all.
Papyrus finds the brother he was looking for. Though, Sans is apparently still young. Everyone, even Naruto, questions this.
(Also Grillby is Sans' age too. I want to make this a Asriel x Naruto and Sansby story)