GitLab packs the whole software lifecycle into one application: repositories, merge requests, first-class CI/CD, security and compliance scanning, package registries and deployment tooling. It is available as SaaS or self-managed, with an open-source Community Edition that many orgs run internally.
It suits engineering organizations that want fewer moving parts and full control over hosting — especially where compliance demands it. GitHub wins on ecosystem and network effects; GitLab wins on breadth-in-one-box and self-hosting.