According to GitLab issue:
https://github.com/gitlabhq/gitlabhq/issues/3788#issuecomment-19044568
Official GitLab doesn't support installed in relative URL. (But it seems not documented)
If GitLab is installed in relative URL, it will cause some problems, like attachment file can't be downloaded.
But Bitnami GitLab is installed in /gitlab , which is not supported. Can Bitnami change it to root URL?