Gonzaga fans are loyal and rabid. They could easily fill the Spokane Arena, but surely you can see how a few more BYU fans would also get tickets than they currently do at the Kennel.
I don't know the rules on these things, but would it be required to be labeled a neutral site game because it's not on their home court? In that case BYU fans could easily fill their 50% or so allotment of tickets. Gonzaga would never do it if that's the case.