View query is using stale=false which means view-engine will update the index before executing the view query. This will take some time since it needs to update the index with all the latest mutations. If its initial build it needs to build index with 754630 items which might take some time depending upon complexity of java script definition.
connection_timeout specifies how many miliseconds your query will wait to receive results before it times out. If query doesn’t receive any response within that period it will give Gateway Timeout.
So it looks like query is hitting connection_timeout since its not received any response in 60 seconds. You could wait till initial index is built completely and then start querying the view.