Migrating repos are straightforward.

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

https://www.youtube.com/watch?v=VYOXuOg9tQI

      At its current state, GitHub importer can import:
      the repository description (GitLab 7.7+)
      the Git repository data (GitLab 7.7+)
      the issues (GitLab 7.7+)
      the pull requests (GitLab 8.4+)
      the wiki pages (GitLab 8.4+)
      the milestones (GitLab 8.7+)
      the labels (GitLab 8.7+)
      the release note descriptions (GitLab 8.12+)
      the pull request review comments (GitLab 10.2+)
      the regular issue and pull request comments
      References to pull requests and issues are preserved (GitLab 8.7+)
      Repository public access is retained. If a repository is private in GitHub it will be created as private in GitLab as well.