GitLab is a web-based DevOps life cycle tool that provides a Git-repository manager providing wiki, issue-tracking and CI/CD pipeline features.
GitLab started as an open source project to help teams collaborate on software development. By delivering new functionality at an industry-leading pace, GitLab now provides a single application for the entire software development and operations life cycle. It also provides everything you need to Manage, Plan, Create, Verify, Package, Release, Configure, Monitor, and Secure your applications.
For GitLab and Git comparison refer to copy - GIT and GITLAB