If you want to remove the branding, you could try to edit the following yaml/rb file, but you need to be cautious about the space as yaml is very sensitive to spacing (no tabs allowed). Also, when you do an upgrade on GitLab, those changes will be lost. Be sure to back up the files before you change anything.
The files are as follows:
/opt/gitlab/embedded/service/gitlab-rails/app/helpers/appearances_helper.rb /opt/gitlab/embedded/service/gitlab-rails/app/views/help/index.html.haml /opt/gitlab/embedded/service/gitlab-rails/app/views/layouts/devise.html.haml
gitlab-ctl stop; gitlab-ctl reconfigure; gitlab-ctl start to apply those changes.
Alternatively, go to /admin/appearance page to setup your logo, title and greeting information.