4. Their tutorial titled "USD Deposit via Ripple" says I need to open a Ripple wallet, fund it with 30 XRP, and then find a nearby Ripple "gateway" that I trust. Does this mean that I really have to trust the gateway to not steal my money? If so, how is this trust achieved?
Bitstamp's a ripple gateway and Bitstamp USDs in the ripple network are really IOUs. When Bitstamp was hacked everyone was worried it would not honour its USD IOUs in the ripple network.
XRPs (ripples) are the only currency in the ripple network that is not an IOU and does not require you to trust a gateway.