GitLab is open source software to collaborate on code.
Manage git repositories with fine grained access controls that keep your code secure.
Perform code reviews and enhance collaboration with merge requests.
Each project can also have an issue tracker and a wiki.
Used by more than 100,000 organizations, GitLab is the most popular solution to manage git repositories on-premises.

GitHub is a web-based hosting service for software development projects that use the Git revision control system. (Wikipedia)

Assembla project workspaces to accelerate software teams, with issue tracking, GIT, SVN, and collaboration