After the de-duplication of the uplink it will use the gateway with the best SNR (and after a certain SNR the best RSSI) for the downlink in one of the RX windows. This set of gateways is stored and in case of Class-C downlink it will use the best gateway again from this set (until you do a new uplink after which it will generate a new set of gateways “close” to your device).