I'm using some Turnkey Linux templates with ProxMox and one is gitlab, but changing the relative root I was still missing some icons when loading the gitlab pages.
According the the post above after updating the config files I needed to go into the gitlab home folder and run:
bundle exec rake assets:precompile RAILS_ENV=production RAILS_RELATIVE_URL_ROOT=/gitlab
This took several minutes to complete (that's how you know its working?) but fixed my problem.