First of all, I want to say thank you GitHub for hosting my website during all those years, for free! With GitHub Pages, it is really easily to make a static site publicly available, and I will definitely use it when needed.
Reason for my moving to GitLab Pages
Despite I have been using Jekyll to build my website, have no plans to change the tool yet, and that GitHub Pages supports Jekyll, it uses GitHub Pages Ruby Gem to build static websites on Jekyll. That means that it is not possible to use a new version of Jekyll or Ruby unless pages-gem supports it. I often update Jekyll and Ruby for my website…
I wish GitHub Pages allowed configuring a build of a site.
I wish GitLab Pages supported CNAME as GitHub Pages does.