Coinbase and GDAX are the same company of course, but they're also separate markets.
GDAX is an exchange so buyers are meeting sellers. All GDAX is doing is hosting and facilitating it.
With Coinbase you're buying from that company and selling to that company. By that definition it's not an exchange. Still, it's interesting that it's never included in any market statistics but perhaps it's not considered to be an active market. They may also want to keep that info confidential.
ok , now i start to understand.
bcause there was something wrong in those statistics...