Coinmarketcap is always using an average price (promedium) between all the exchanges that have bitcoin listed on their platforms.
Most of the times the rates especially for BTC is the highest I could find somewhere.
And no, gdax is probably the only exchange that has bitcoin over $800 - $1000 of the actual price.
The same happens with BitHumb, when there is too much volatility, the price between it and the other exchanges are far more than 10 or 12 percent.