I run GP on tons of sites and wanted to post the owner’s response in case anyone else was wondering (just FYI).
Hi there, as long as they stay compatible with WordPress themes and plugins, GP will be compatible. It’s not something we’ll go out of our way to support at this time though – GeneratePress is a WordPress theme, and we’re excited about the future of WP with the new editor.
I use GP on several sites too. I think what they’re saying is, as long as CP doesn’t deviate from the way WP functions, there will be no problems.
In other words, unless CP starts doing things in a non-standard way, GP will work as normal. Not a technical answer, but I can’t see any reason that should happen in future versions. At least not in the foreseeable future.
I don’t know if this has been discussed, but I’m wondering if it ever becomes an issue, if CP could make a plugin to bridge any theme/plugin gaps to ensure 100% compatibility.
The team will address issues as they arise. They are very busy making the Beta version stable and working on 1.0.0
They aren’t looking that far ahead yet. They have to focus on current releases.
Yep - just thinking out loud w/ my morning Joe 
Yes, this is an option. Wherever possible, though, we’d prefer to either just remain compatible in the first place, or encourage plugin and theme authors to do things in a way that are compatible with WP 4.9.x and ClassicPress.