A gateway is some business that has agreed to issue and redeem ripple IOUs. You take your money to a gateway and the gateway gives you a ripple IOU. You can send this IOU to other people, trade it for XRP or BTC or whatever. You can take your IOU back to the gateway and the gateway will send you money. This is essentially what paypal or dwolla or banks do now.
I don't think this is the same what paypal or dwolla or banks do. Why do you think, say USD IOU issued by gateway A will always be in par with USD IOU issued by gateway B?