[feature]: Enable Full GitLab Integration with Issue Sync (Parity with GitHub Integration)

Is there an existing issue for this?

• I have searched the existing issues

Summary

Currently, Plane supports a robust GitHub integration that allows synchronization of issues between Plane and GitHub repositories. However, the GitLab integration is limited — it does not currently support automatic syncing of issues, comments, or status updates between Plane and GitLab.

This limitation significantly reduces the usefulness of Plane for teams using self-hosted GitLab or GitLab SaaS, as they cannot maintain alignment between their Plane projects and existing GitLab workflows.

Why should this be worked on?

• Improves adoption among organizations using GitLab for source control.
• Ensures a unified project management experience without switching tools.
• Reduces duplicate effort and human error from manual synchronization.
• Brings GitLab integration to functional parity with GitHub integration.