It may have been because he initially used gh-pages and the page layout flexibility wasn’t present in GitLab at that time.
Gitlab pages are not activated on gitlab.kitware.com and will not be for the time.
A simple solution is to have a github mirror that host the gh-pages.
(That what we did for F3D : https://kitware.github.io/F3D/)
I keep getting prompted for my Kitware, (not GitLab) username and password.
We had a similar issue with F3D, it can be fixed locally easilly.