The two features I really wanted when I started Gutenbrg were Sass compilation and syntax highlighting built-in: a clean build of my blog with Hugo + Pygments was taking ~20s for ~20 pages at the time and needed virtualenv for Pygments, Node.JS/npm for gulp-sass and the Hugo binary. Hugo now has syntax highlighting in Go so that part should be better though.
Themes are supported in Gutenberg but there is only one so far as I didn't want to spend time on that yet. It is pretty fast to port themes though - took me about 10 minutes for Hyde: https://github.com/Keats/hyde - so I might port ~10 or so next week to get the ecosystem started.