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An Issuing Gateway receives money (or other assets of value) outside of the XRP Ledger, and creates issuances in the XRP Ledger.
This provides a direct way for customers to get money in and out of the XRP Ledger.
All currencies in the XRP Ledger, except for XRP, take the form of issuances tied to a specific issuing gateway.

https://ripple.com/build/gateway-guide/