I think you didn't get it right, Ripple gateways are not equivalent of Bitcoin mining nodes. They are equivalent of bitcoin addresses. Obviously you have to make a decision to
Transfer funds to a business entity. Running a gateway is not necessary for all users but it's required for bigger businesses running on Ripple, like Bitstamp.
Anyway comparing it to PoW is not that much useful. Obviously PoW is more secure but it's also more expensive and is created to address a different usecase
