1. Go to `Settings > Repository > Mirroring repositories > Add new`
2. Fill the form :
    - Git repository URL : `ssh://github.com/[username_or_org]/[repository].git`
    - Detect host key
    - Authentication method : `SSH public key`
    - Username : `git`
    - Submit
3. Copy SSH public key (copypaste icon)
4. Add SSH public key to your GitLab / GitHub profile
5. Sync