You probably know that Coinbase and Gdax is the same platform. Gdax positions your self how more professional exchange.
Is it supouse to be same prices ?
Usualy it is the same. But at a time you need it most, they are different !
And to addition to that coinbase wont let you to send you coins to other address. Right in time you need it !
Be carefull using it !
Thank you for reading !