There are indeed quite a lot of open PR on bitbucket, and writing a script to automatically migrate them on GitLab would have taken too much effort. There are two options:
1 - either the PR author, or anyone else, redo the PRs he/she care about on gitlab by hand and close the bitbucket PR with a respective link (through raw diff/patch files, or maybe git-remote-hg if this tools can fetch selected changesets)
2 - for simple PR that are unlikely to produce conflicts, we can also continue the discussions on bitbucket and once ready an Eigen maintainer will do the merge by hand (we used to do that quite often anyways)
Gael