There are several instances of gutenberg CSS classes which need to be renamed for core.

.gutenberg-editor-page should become .block-editor-page.
.gutenberg should become .block-editor.
.gutenberg__editor should become .block-editor__container.

There are quite a few references to these classes in the docs, they'll also need to be fixed.

Core will use both the old and new class names while this issues is fixed.