I was just needing this the other day. Here’s a snippet that should work for enabling a notification test mode. You can then toggle the $intercept variable.
add_filter( 'gform_notification', function( $notification, $form, $entry ) {
$intercept = true;
if ( $intercept ) {
$notification['to'] = 'email@example.com';
}
return $notification;
}, 10, 3 );