In order to enable Shibboleth support in gitlab we need to use Apache instead of Nginx (It may be possible to use Nginx, however I did not found way to easily configure Nginx that is bundled in omnibus-gitlab package). Apache uses mod_shib2 module for shibboleth authentication and can pass attributes as headers to omniauth-shibboleth provider.
exclude shibboleth URLs from rewriting, add "RewriteCond %{REQUEST_URI} !/Shibboleth.sso" and "RewriteCond %{REQUEST_URI} !/shibboleth-sp", config should look like this:
1. Edit /etc/gitlab/gitlab.rb configuration file, your shibboleth attributes should be in form of "HTTP_ATTRIBUTE" and you should addjust them to your need and environment. Add any other configuration you need.
On the sign in page there should now be a "Sign in with: Shibboleth" icon below the regular sign in form. Click the icon to begin the authentication process. You will be redirected to IdP server (Depends on your Shibboleth module configuration). If everything goes well the user will be returned to GitLab and will be signed in.