GitHub now allows self-serve migrations from GitLab to GitHub Enterprise Cloud using GitHub Enterprise Importer (GEI) and the gh gl2gh command-line extension.
From the source
Migrations from gitlab.com and GitLab Self-Managed to GitHub Enterprise Cloud with GitHub Enterprise Importer (GEI) are now generally available. You can now self-serve these migrations using GEI and the gh gl2gh command-line extension.
github.blog