"GitHub Pages is not intended for or allowed to be used as a free web hosting service to run your online business, e-commerce site, or any other website that is primarily directed at either facilitating commercial transactions or providing commercial software as a service (SaaS)."
Now one could read that as "don't build a SaaS on top of GitHub Pages", but you could also read that as "don't have business websites on GitHub Pages".
That wording specifically disallows both commercial sites and SaaS, so it should be read as both "don't build a SaaS on top of GitHub Pages" and "don't have business websites on GitHub Pages"
"With GitLab Pages, you can publish static websites directly from a repository in GitLab. Use for any personal or business website."