When I needed my Pitbull dog to gain weight I fed him Bully Max High Performance High Protein Dog Food and it helped him gain weight pretty quickly.
It's a bit expensive at $45.00 per bag but it's well worth it and did work to help my Pitbull gain the needed weight then I switched him back to some Taste of the Wild Grain Free dog food which is only $30.00 per bag online.
Purina Pro Plan Sport Dry Dog Food is another good dog food for Pitbulls to gain weight too.