Timely post. Keep in mind Coinbase did NOT distribute BCH to its bitcoin-holding customers last summer, so they will now be making those distributions which might incite a LOT of selling by holders who think others will sell too-- a "prisoner's dilemma" type situation.
That is the "yang"; the "ying" is that adding BCH to GDAX exchange will mitigate some of this selling so long as there's still strong fiat money inflows into Coinbase. Fiat money inflows come from financial institutions putting their first dollars/yen/euros into Coinbase and making their first crypto investments-- which can now include BCH.