XRPs are a native currency, gateways don't issue and redeem XRPs.
On the other hand, gateways issue and redeem their own IOUs (usually denominated in a fiat currency such as USD or EUR).
At a minimum Ripple should incorporate the existing banking system, and also do much much more. I would love it if I could open a checking account that lets me hold not only US dollars but also Bitcoins, XRPs, even Euros, and when I write a check or use my debit card it sells just enough of my non US-dollar assets to cover the cost. This is the holy grail, for me at least, because then I don't care if Amazon accepts Bitcoins or uses Ripple, I can just pay them using my debit card like normal except that the money will come from a just-in-time sale of Bitcoins on the distributed Ripple exchange.
The promise of Ripple is that with a Ripple gateway bank account, you can spend your bitcoins anywhere that takes credit cards, checks, ACH payments, or wire transfers.