GraviTale is a crossover AU, a mix of Undertale and the Flipnote Hatena series Gravity by Vicky. (Find more info HERE)
Every character is replaced by their Gravity counterpart.
Flowey is replaced by Devyn, the assumed antagonist in Gravity.
Toriel is replaced by Claire, who is [spoiler] Emma and Molly's mother in Gravity
Sans is replaced by Hector, friends with Emma, Chase, Kim, Mio, Devyn (in the future) and the two twins, Nina and Joel.
Papyrus is replaced by Mio, friends with the gang, however, she's closer to Hector. (Leave out the fact that they end up dating and the pill will be easier to swallow. Might rework the plot so it's more fitting)
Undyne is replaced by Emma, the main protagonist of Gravity.
Monster Kid is also replaced by Molly, who's Emma's younger sister.
Alphys is replaced by Chase, who's Emma's love interest throughout Gravity, also one of her closest friends.
Mettaton is replaced by Kim, who's Emma's best friend throughout Gravity, and basically the gossip queen of the group.
Asgore is replaced by Luke, the father of both Emma and Molly.
Chara is replaced by Lucy, the main antagonist in Gravity.
I will edit this later and lay out a more detailed plot. For now, I hope this is enough information to keep this page up.