I've kinda wondered that too but, i was kinda glad that he actually pput up a fight. However if the fight had dragged on any longer with shukaku at full power gamabunta would have been in big trouble.
But Gamabunta also fought with the 4th hokage against the kyuubi, so he must be quite strong, it probably depends on the summoners skill too how well they peform. And i slaos think it was slightly because shukaku is the one tails...but that doesn't mean he can be defeated by any skiled ninja...