The versions of GravSans
There are several instances of GravSans at any given time. The 2 most notable are GravSans and DoomedSoulGrav.
GravSans - The happy gravity-manipulating Sans you all know! (Hates the death/injury of other Sanses) Hates talking about the ‘incident’
DoomedSoulGrav - A Sans that got human souls and was catapulted backwards in time. He lived 4 years after the destruction of his universe and was thrown backwards in time, along with his version of Frisk, to escape the Purger. In a last ditch effort, he came back in time to kill FractureSans and use the Fractured Blaster as bait to get his past self to kill the Purger.
SoulGrav - At some point in his timeline, Grav went insane from the resets in the universe he had come to call home. He went on a bout of insanity and almost killed his best friend, who tried to stop him from using DETERMINATION. Shortly after she succeed, Sans took the 6 human SOULS, almost killed Asgore and took his trident. And almost killed Frisk. (The exact event will appear on the AskGravSans! blog when it reaches that point in time.)
Anti-FreshGrav - This version of Grav has been partially infected with the Fresh and has come to hate it so much that he actively seeks and tries to kill Fresh and all of his doppelgangers
InsaneGrav - This version of Grav went down the same path as SoulGrav, but succeed in getting the DETERMINATION and killing Frisk. He roams aimlessly around the multiverse indiscriminately killing.
If you want to know about the incident, ask GravSans a few times on the ask blog. (I’d link it if I knew how.)