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In unscheduled outages we see connections fail or time out most of the time, and sometimes HTTP status 503 with a message like "Our servers are currently too busy to handle your request. Please wait a minute and resubmit. Thank you." In scheduled downtime it's usually just failed connections.
If you wanted to send in a harmless ping to see if the gateway is back online I suppose you could send a authorization request with x_Test_Request set to TRUE.