Not sure, it's going to be a fucking mess that is for sure, bitcoin cash is either going to explode on every other exchange or eat shit on GDAX real quick..
I saw the same thing you did...
https://steemit.com/bitcoin/@pawsdog/coinbase-wow-arbitrage-anyone
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Yeah I saw that too. If you read the article, basically there were no sell orders as everyone wanted it.. so the massive buys crushed the order book and there are 10's of millions of dollars stuck way high in price. Especially when the price has not changed that drastically on the other exchanges. As I said I be there are a shitload of people sweating bullets right now with a 5000 plus dollar BCH coin in the wallet.. When GDAX comes off post only its going to be sell city.. I may put in an order around 2500 to see if I can catch a flash crash.. It's going to be a mess for sure...