As of this writing, GitHub Pages still doesn’t support HTTPS for custom domains. Last week, I came across Netlify being able to serve GitHub Pages as is, with HTTPS support even with custom domains. I gave it a go. And it worked. If you’re in the same boat as me (using GH-Pages with custom domains), give Netlify a go.
Note: I am not affiliated with Netlify in any way, whatsoever.