For many years I used Jekyll (back then I used octopress - a Jekyll framework) - now I use node and git wikis - but that would not help you
Hugo and Gatsby produce the same end result. I hope Gatsby is better then Jekyll (thinking of playing with it soon).
I write my posts in Sublime Text
(Don't read too much into that setup just admire the sexiness of typing in your favorite editor)
I don't believe you'll find a better GUI then your favorite editor.
My workflow right now (and I'm sure Gatsby offers a similar thing), in terminal:
node create.js web sept-day-03 diary sep-20
And just start typing!
Then in terminal I push the changes and done.
Much easier then any thing else that would have you navigate to it, login, and use their crappy rich editor.
Then you push those changes to github, and github pages displays it for you
As with everything you need to do once in a new moon, setting up git pages and CNAME, you'll need to ask google but after that it's just typing
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