Gargantus is certainly a strong robot. His physical power is on a whole different level and even Ironman’s energy blasts had a hard time hurting him. At least, that was the case for his old armor. With one of the many modern ones at his disposal Tony will have no trouble taking Gargantus down for the count. The beast is far too slow to do anything about it so Ironman will win without a scratch. Ironman wins.