Hi there,
1. Regular CSS styling isn’t loaded in Gutenberg. We’ve specifically taken the content aspects of the theme styling (things you can customize), and integrated them into Gutenberg using Gutenberg-specific CSS selectors. In most cases, the selectors you’d use on the frontend of your site wouldn’t apply in Gutenberg.
2. To remove the block editor styling that GP adds, you can do this:
remove_action( 'enqueue_block_editor_assets', 'generate_enqueue_backend_block_editor_assets' );