Turn off Wordpress Gutenberg editor for your custom post types

functions.php

add_filter( ‘gutenberg_can_edit_post_type’, ‘can_edit_post_types’ );
function can_edit_post_types( $can_edit, $post_type ) {
    If ( in_array( $post_type, array( ‘a_post_type’, ‘another_post_type’ ) ) {
        return false;
    }

    return $can_edit;
}