GitLab is a Ruby based web application that manages Git repositories combined with issue tracking, wiki and other team/collaborative functions. It's a "GitHub at home".
Please consult the first installation guide.
After each upgrade of minor version (eg. 8.11.x to 8.11.y) please consult the Update guide for patch versions
After upgrade of a major version (eg. 8.11 to 8.12) please consult the Major GitLab upgrade guide