If not fees, what incentive would someone have to be a gateway? I can't really think of much reason for a company to become a gateway.
So if gateways will charge fees, doesn't that diminish the value of the system? What cost advantages would it have over the money transfer services available now?
They will almost certainly charge fees unless they're fractional reserve or have some other incentive, see
https://ripple.com/wiki/Transit_Fees . If it all works as envisioned then the system will be more open than what we have which should allow for tighter competition, hence lower fees.