Taming a Gigantoraptor with another Gigantoraptor DOES give bonus taming levels IF you manually leveled it up. The bonus taming levels gained are based on the x/88 levels you put in. So to get 75 bonus levels on a lvl150, you need to level it 88 times. Oh, and you need to own a Gigantoraptor that is higher lvl than the baby you want to tame for it to work. Same goes for other baby dinos.