I've built a number of custom blocks with ACF. You have a choice of modes for showing your block in the editor: edit and preview. This works a lot like the core HTML block, except that the "edit" mode shows you the custom fields. If you switch to the preview, the fields appear in the sidebar. I imagine trying to work in the sidebar for something like a gallery would be awkward.

I'll have to download the plugin and see how it's set up and how easy it would be to separate a few of the more interesting blocks, as I already have too many block libraries installed and wish more people would develop not only individual blocks, but blocks that are not in the popular collections.

The dependency on ACF Pro wouldn't be an issue for me, since I use it on almost every site I build anyway, and there are sites that I'm switching over from ACF-based templates to Gutenberg where it seems like a very natural fit.