Until Eternity by SinisterChic (aka Heather Martin) and on ff.net

Summary: Complete! 55 yrs after The Gift Spike is working for the council. The next slayer he comes across seems too familiar

The Jumpverse by OffYourBird starts withn A Different Kind of Hell. Buffy and Spike both jump off Glorys tower and end up in Victorian London.

https://archiveofourown.org/works/11689914/chapters/26317029

And one more:
Consequences by u2fan2005

Summary: Post- The Gift. Buffy's friends perform the spell to bring her back to life...but there was a side effect. Buffy finds herself crawling out of her grave 80 years after the spell was perform. The world around her is now very different and there is only one person she recognizes...or maybe two....

Sorry, I had already made that rec... 😶