Strength in numbers. Sure, he can deflect the first couple gallerhorn blasts, dodge an onslaught of nova bombs, and shake off bullets like dead skin. He can't keep it up forever, he will get tired, and when he does; the hits will start taking their toll. Guardians are tireless machines of war, and there's a lot of them. Goku can't constantly defend himself, no matter how many levels of super sayin there is. He can't stop a re spawning army of guardians.