Disable Gutenberg by Template (example)

/**
  * Example of disabling Gutenberg by template
  * @see https://www.billerickson.net/disabling-gutenberg-certain-templates/
  */

add_filter( 'gutenberg_can_edit_post_type', 'disable_gutenberg_editor_by_template', 10, 2 );

function disable_gutenberg_editor_by_template( $can_edit, $post_type ) {

    if( !is_admin() || !isset( $_GET['post'] ) || !intval( $_GET['post'] ) ) return $can_edit;
    $exclude_templates = get_option( 'my_array_of_post_types_to_disable' );
    if( !$exclude_templates ) return $can_edit;

    $current_template = get_page_template_slug( $_GET['post'] );

    return !in_array( $current_template, $exclude_templates );

}