For 504(Gateway Time-out) http status code apigee is not sending custom response as per Fault Rule Handling, instead it sends html response, even though in trace window under Response Sent to Client, it shows expected response which is as per fault rule handling. 

But somehow when we see the final response on Postman, we get a html stating Gateway-timed-out .

We have also tried to limit timeout with below property with no luck
<Property name="io.timeout.millis">30000</Property>
<Property name="connect.timeout.millis">5000</Property>

Can you please help to debug this issue.