Hiya,
The Gutenberg project (which is actually a revamp of many areas, the editor was the first phase) will keep moving forward, there’s no changing that.
I’m not familiar with WPPage Builder, but if it’s the one I’m looking at, they claim to be compatible with the new editing interface introduced in WordPress 5.0, so this shouldn’t be a problem using it. If you are experiencing issues with it after this I’d recommend reaching out to them to see if it’s something that can be fixed.
As for when any potentially found bugs are resolved, that depends on the bug, if it’s been reported yet, and how it aligns with upcoming releases, it’s hard to give an exact date but you can follow along with tickets for bugs and enhancements on the project page at https://github.com/WordPress/gutenberg/issues