How can I use Auto DevOps progressive deployment to migrate a small application without downtime in GitLab?

A small monolithic application hosted on GitLab needs to be migrated to a new version with zero downtime. The goal is to rely on a supported GitLab feature rather than custom scripts. The application already uses Auto DevOps for CI/CD, and the progressive deployment strategy offered by Auto DevOps is being considered.

Questions:

• How can progressive deployment be enabled and configured within Auto DevOps?
• What environment variables or settings are required to control the staged rollout?
• How can the migration be verified to result in zero downtime?