In reply to Andre.

Yes, Gutenberg is the testing for what will land in core.

My approach for theme development would be to code against the latest version of the plugin. Then, do final compat checks against core and the classic editor if you want to support it.

With full-site editing, you're safe for a while. From the roadmap I've seen, they're shooting for a basic working version by the end of 2020 within the plugin. I imagine it'll be sometime in 2021 before it is feature-complete enough to be in core.