While setting up this site, I struggled to get SSL/HTTPS operational through GitHub Pages using Google Domains as the NameServer. GitHub, still doesn’t enable the Enforce HTTPS option, but my site is very much protected thanks to CloudFlare’s free NameServer Hosting that automatically enforces SSL. The options were fairly clear during setup, but if you want more information about how/why this works, I found out about it from 
Adam Brown’s blog.
Note, that CloudFlare will automatically import any DNS stuff you’ve already setup on Google Domains.
I’d also point out that Adam’s GitHub Pages theme looks setup well if you want ideas.