He has a decent shot at a .725 - .750 OPS and and a .320 wOBA. He's as likely to beat that as he is to underperform it. That makes him around an average hitter, even though it is put together in a weird way. If he's a dog, he can be released.
Look, I'd have preferred Pederson or Rosario, and would have taken Belt if he would have signed here (Votto too). I suspect all those guys get more than Gallo. He was an affordable dice roll / lottery ticket.