How does a gateway IOU differ from any other IOU on the Ripple network? Say, if I got a $1000 from a gateway, does that make it a better currency than say, creating a second account and sending myself $1000 through that? Does the Ripple network somehow distinguish between those?
It doesn't differ.
Gateways are just issuers that you somehow have more trust for, so much trust that their IOUs can basically be treated like cash.
This is what happens in the current financial system, when you see $100 on your bank account it doesn't mean that you actually have $100. It just means that you hold your bank IOUs.
From what I understand there is no technical distinction between both types of IOUs.
In my understanding, the difference between a gateway and any random user is that a gateway has a well-defined process for getting non-Ripple stores of value into and out of the system. Users have to register with the gateway, and then they can put money in or get money out. So if you have some random IOUs, they probably won't be worth much because anyone wanting to redeem them will have to track down the random person and convince them to exchange their IOUs for actual items of value. But if you have a gateway's IOUs, they'll probably be worth close to par, depending on how streamlined the process is of getting the real value.
If a gateway stops processing redemptions, it's no longer a gateway.