How Ripple actually transfer money from one bank to another? Imagine there is bank A from USA and bank B from England, and they are both on Ripple Network.
Bank A wants to transfer $100 to bank B.

How does bank A put money in the Ripple network? Does it need to buy XRP from some gateway?

Suppose bank A already has the equivalent of $100 in XRP.
Bank B then receives XRP, but bank B is not interested in XRP, it wants pounds.

How will bank B convert to pounds? Does it need to find a gateway?
What if the exchange rate at this gateway is terrible for him?
I think it's necessary to first consider the question: Without ripple, how do banks "actually transfer money" from one bank to another?

Suppose that you and I are banks, and I want to send you $10 million. I'm probably not going to actually physically ship you $10 million worth of bills and coins. Instead, there are a few ways we could do this:


You and I privately agree that I now owe you $10 million. I don't send you the money, just an IOU. Next month you might want to send me $8, and so you just cancel $8 million of that IOU. We keep track of who owes who how much, but might never actually need to resolve it, since in the long run we're both sending a lot of money back and forth. This is difficult, however, because every bank has to maintain debt with every other bank.
You and I make an arrangement through a mutually trusted third party. I agree to owe them $10 million, and they agree to owe you $10 million. Later, someone else might send me $10 million the same way, negating my debt to the third party. A "wire transfer" in the US is just this, where the third party is the Federal Reserve.
You and I announce to a trusted third party that $10 million of my money should now be considered yours. This third party isn't a middleman who holds debt, like the central bank described in #2 - it simply keeps track of IOUs and regularly announces to all parties who owes who how much. The ACH system in the US (a.k.a. "Direct Deposit") works this way.


So, for the most part, bank's don't actually send money to each other at all -
 they just keep track of who owes who how much. Actually settling those debts, if it happens at all, has to happen outside of the systems above.

Ripple, as I understand it, can be thought of as a decentralized version of #3. It's basically a distributed public ledger of IOUs. (And those IOU's can themselves be traded as currency, when they're written by a trusted party.)