Can I be held liable if a gateway vanishes?
You are not liable when a gateway vanishes.
When you send money on Ripple, you are trading IOUs. In order for someone to receive IOUs, this recipient must have expressed his acceptance of IOUs from the corresponding issuer.

Alice wants to buy a lot of ice-cream from Bob for $20. They both trust the gateway Gateway.
Alice has previously paid $100 to Gateway and holds therefore an IOU from Gateway with the value of $100. She sends $20 to Bob. Bob also trusts Gateway, so he is willing to accept Gateway-IOUs as payment. Gateway now owes her only $80, and Gateway owes $20 to Bob.

At this point their deal is done. So, if Gateway went under after this, there would be no liability, because the payment was transferred and accepted by Bob. His beef is with Gateway, or with himself for trusting Gateway, not with Alice.