The fact that everyone already knows what Coinbase is and now they're throwing away their entire marketing, image, and name recognition is probably the dumbest corporate move I've seen in Bitcoin so far. Who signed off on that? Is it a unilateral Brian Strongarm move?
It's only the exchange that is being named GDAX. The main wallet is still coinbase.
As for why they are doing it. The best explanation I saw was the following:
Previously: Coinbase - A Bitcoin Payment Processor, Storage, and Exchange
Now: Coinbase (GDAX) - A Digital Assets Payment Processor, Storage, and Exchange
They arn't sure that bitcoin will survive so are rebranding for cryptos in general.