This is a very different situation from the DAO hack you're probably thinking of. Most of the losses were off the blockchain, in GDAX exchange accounts.
I actually don't see how its different. This was caused by an individual resulting in people losing money. The question then becomes how much lost money or WHO lost money is enough reason to pursue a hard fork?
Then GDAX should do this, they have their internal market.
The difference is the DGAX trades were off-chain (on GDAX servers). A hardfork wouldn't change anything.