I couldn't really find this on their site, what's the difference between the two? Why did they fork, are there ideological differences or something? I'm not using gitea yet, but was considering it.
Those questions are covered in the FAQ!
How easy is it for them to turn their back on "Forgejo will always be Free and Open Source Software."?
As far as I know the plan is to upstream it to Gitea as well, fwiw