I'm working on this, have a look at https://github.com/MichaelMure/git-bug

you can import github projects including issues and stuff into gitlab

https://docs.gitlab.com/ee/user/project/import/github.html

Some thing would be lost, yes [1] but the core code and commit history would be preserved. Issues can still be re-opened if still relevant, and pull requests can still be seen in the commit history. Comments on pull requests, if not approving a pull, will result in additional commits to the code which will be preserved.

Whether this is OK for a given use case depends on how important ticket and pull request comment history is. For open source projects, I don’t consider that kind of history nearly as important as the code itself, since a lot of issues are wishlist requests, support requests, or downright spam.

[1] Including branches unless one runs a script to go to and clone each branch to the new repo