You can import your existing GitLab.com projects to your GitLab instance, but keep in mind that it is possible only if GitLab.com integration is enabled on your GitLab instance. Read more about GitLab.com integration for self-managed GitLab instances.
To get to the importer page you need to go to “New project” page.
Go to the Import Projects tab, then click on GitLab.com, and you are redirected to GitLab.com for permission to access your projects. After accepting, you are automatically redirected to the importer.
To import a project, click “Import”. The importer imports your repository and issues. Once the importer is done, a new GitLab project is created with your imported data.