I run GP 2.2.2 and GPP 1.8.1 on WP 5.1.1, and have the Gutenberg plugin running, version 5.6.1.
I am running the Gutenberg plugin because I’m writing a manual and want to have the latest version on hand.
When I have the Gutenberg plugin off or an older version running, the editor is styled as it should be, with the headings in the colors and size I selected. With 5.6.1, the headings are the default for the editor, no styling.
According to the WP Slack channel, this may be because of older rules in GP:
The specificity required to style the editor has been reduced so it should, going forward, be simpler, but if an older syntax was used it could be off.