GitLab powers the Stoplight backend, including (git) file storage and relational data.
GitLab requires persistent storage in order to store Stoplight file data (in Git), and optionally PostgreSQL data (when using the omnibus package).
Packaged within the GitLab CE is an embedded installation of PostgreSQL and Redis. These two sub-components can be broken out into external services if needed. You may also break out these services if you plan on using a managed hosting solution, for example, Amazon RDS (for PostgreSQL) or Amazon ElastiCache (for Redis).