Yep! At the most basic but most… blunt, is just git rebase, but that’s a sledgehammer that, although would work, can be a bit ‘overwriting’ of the current history state, lol. There are other more fine grained methods though but more work.
Oh, and make sure to back up your git repo entirely, you can corrupt it all by messing with history as history is designed to be immutable. Would probably be safer to fork GT6 into a new branch, apply your main history on top of that (standard merge it in), then change that branch to be the main branch in github.