If you have "one big gateway", it's always going to be a single point of failure. One thing Ripple does to reduce this risk is not allow gateways to freeze individual balances. So if a gateway refuses to redeem for particular people for any reason, so long as they're still in operation generally, those people can exchange balances at that gateway for some other asset.
I'm talking about the scenario where specific individuals who are citizens of the United States have a blanket ban on deposits to their accounts / frozen accounts. This would affect
all gateways which provide settlement via ACH or wire transfer. In theory these individuals could do a cash settlement (gold or fiat). Still, there would be a blow to confidence.
But it's certainly true that some kind of general failure of a major gateway could be a significant blow to the Ripple network. This is one of the reasons we are working to get some gateways going that are operated by regulated and insured financial institutions.
This is definitely helpful but the banking cartel could always use government force / regulatory capture to disrupt the smaller banks. Most likely this will be done under the ruse of preventing money laundering, stopping terrorism, or hindering child pornography. A court order might prevent ACH / wire settlements at smaller depository institutions, creating panic and driving the value of those IOUs to zero. The result would be a flight to the "anointed" banks (Goldman Sachs, JP Morgan, etc...)
Technology works both ways. It allows us to be more productive but it also allows those with a monopoly on the use of force to more efficiently extract output from the rest of the world.