Writeup here: https://daiyi.co/blog/pr-previews-for-github-pages/
A feature of modern hosting platforms (e.g. Vercel) that I miss with gh-pages is getting automatic site previews on each PR. Here is my attempt at a hack.
It works by using github actions to build the site on each commit to a pull request, and pushing the result to the /pull/{pr-number} folder on the gh-pages branch. This means the preview can be accessed at https://github.io/username/project/pull/{pr-number}. When a PR is closed, another action deletes the folder.
Live demo of an open PR with a site preview, and a closed PR with preview cleaned up.