WordPress: Allow Gutenberg for specific user roles

functions.php

<?php

add_filter( 'use_block_editor_for_post', 'my_prefix_maybe_allow_block_editor', 20, 2 );
/**
 * @see https://core.trac.wordpress.org/browser/tags/5.1.1/src/wp-admin/includes/post.php#L2093
 *
 * @param bool   $use_block_editor
 * @param string $post_type
 * 
 * @return bool
 */
function my_prefix_maybe_allow_block_editor( $use_block_editor, $post_type ) {

	// if you want to disable block editor for specific post types
	$use_block_editor = in_array( $post_type, [ 'post', 'custom-post-type', // .... ], true );

	// disable block editor if logged in user isn't an Administrator (user role)
	// @see https://codex.wordpress.org/Function_Reference/current_user_can
	$use_block_editor = current_user_can( 'administrator' );

	return $use_block_editor;

}