You access Gerrit Code Review from the Code Review tab in the GitCentric GUI. Log-in and registration are handled through the GitCentric GUI. See Log In to GitCentric.
If you are familiar with Gerrit Code Review, you may notice the following differences from a standalone Gerrit Code Review installation.
Top-level menu items take you to the GitCentric GUI:
Otherwise, all other Gerrit Code Review functionality is handled through the Gerrit UI, and documentation for Gerrit features is handled by the Gerrit on-line help. Gerrit Code Review has its own documentation, which you can find here: http://gerrit-documentation.googlecode.com/svn/Documentation/2.7/index.html