Payment gateway routing is the set of rules inside a payment gateway or payment orchestration platform that decides where each transaction goes next: which acquirer, processor or local payment method handles it. It is the merchant-level layer of payment routing, which also covers network and bank-rail choices and is explained in full in the main entry.
Rules can be simple, for example sending all euro cards to one European acquirer and everything else to another, or dynamic, using live data on approval rates, cost and uptime. The dynamic form is usually called smart payment routing, and choosing the cheapest path is least cost routing.
Gateway routing depends on the merchant having more than one processing relationship, as in multi-acquirer processing. It also supports payment failover, where a transaction is resent to a backup provider when the first is down. Routing between debit networks on a single card, as required by US rules, happens later in the chain and is not usually controlled by the gateway alone.