I understand plugins aren’t ready for Gutenberg yet. I just created a CPT. To test I chose Add New from the admin menu. At first all of the custom fields displayed. I clicked one of them, and all disappeared except the first for the generic Add Title. The page is now treating this as a standard post.
After that I tried page refreshing and getting in from Add New : Selecting Add New Gutenberg had the same result. Selecting Add New Custom Edit was also showing entry fields for the default post type.
To summarize. There’s a CPT with fields now but no way to add new records.
Deactivate Gutenberg, and all fields for the type display.
Reactivate Gutenberg and the same as above occurs – see the fields, they disappear, no way to recover them.
I can’t tell if this is a Gutenberg issue with CPTs, or a CustomPress issue with Gutenberg, sorry.