They're both boys right? It often happens that if you get two same sex gerbils that aren't littermates, they will dislike each other. It'd be a better idea to keep them in separate cages/tanks if they're biting each other. Once a pair of gerbils decide to fight, they fight to the death, rolled up in a ball, going for each other's necks. Not something very nice
Google's disability is probably part of the problem as well - he can't see where Pumpkin is and he probably gets a shock when he realises where Pumpkin is. Then he might lash out because he's surprised.