Is it possible to run a Kirby CMS website hosted in GitHub Pages?
No, you can only host static sites: https://docs.github.com/en/github/working-with-github-pages/about-github-pages#about-github-pages
What you could do is run Kirby locally and use the static site generator plugin. https://getkirby.com/plugins/d4l-data4life/static-site-generator but obviously you won’t have a panel on the github pages site
OR! you could do the insane thing and run the build process on GitHub as well! I figured it out after a while and wrote a post about it. It has caveats, but it works: