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Use Stackbit to create your combo of SSG, Theme and CMS. https://www.stackbit.com/

Stackbit really is a great product! I was able to throw up a marketing site for my family's rental property in a few hours for some non-technical people to manage.

Forestry.io is a nice wrapper around a bunch of SSGs, backed by git. It's a paid subscription, but with a pretty generous free tier. I'm using it so my less-technical co-founders can edit our Jekyll-based support site.

Jekyll has jekyll-admin: https://github.com/jekyll/jekyll-admin

Forestry.io supports an entire mess of site generators including Hugo and Jekyll. You can also leverage Github or Gitlab's ui directly to edit or create files.

publii has a very well polished GUI https://getpublii.com/