Besides the lulz, shorting GavinCoin (especially with element-of-surprise proportional leverage provided by NotXT) can make a lot of money.
The problem is that, as I tried to explain, there will not be a "Gavincoin" and an "Adamcoin". Even if the chain splits, and the 1 MB branch survives for a while, there will be just a two-headed bitcoin. Even if you could trade and speculate on each coin separately, you cannot attack one in the market without harming the other.
If the fork is not successful, all transactions on that fork will be void; simple orphans from a (1 MB) Bitcoin point of view. That happens every day, it's why we wait for more than one confirmation.
If a person decides to take the risk and accept the fork, don't you think that person would also want a premium for assuming that risk?
If I have a claim in Bitcoin and the other side is trying to settle with XTCoin, how do they pay the premium even if I were to accept?