are the core developers and other contributors migrating to Forgejo?

This is my question as well. If the majority migrate then I will follow.

Some of them have migrated. I imagine others are taking more of a "wait-and-see" approach.

For what it's worth, Codeberg is quite a major contributor to Gitea as well as the biggest public instance, and they are supporting the fork and will be switching over after the first stable release.

why fork gitea and not gogs? is gitea "better" than the former?

Yes, you can see a comparison on Gitea's website. The Gitea community has put substantial development into growing Gitea into becoming a GitHub/GitLab alternative. I hope Forgejo will take it further, or that Gitea will actual work to become a community project like they promised.

The people starting forgejo were working on and with gitea before. With that I suppose the question is "Why fork anything but gitea?"