I think that when you buy a venomous animal its aggressiveness or lack thereof isn't really a consideration because you're handling it in such a way to make a bite impossible, or at least to the best of your ability. At least that's the way I always look at it. But to tell you the truth I don't think feeding live prey makes reptiles aggressive. One of my most docile snakes (rainbow boa) eats nothing but live and my most aggressive (western diamondback) eats dead.
:I wasn't sure if they used dead because if they used alive the Gila may get more aggressive or not.