then, check the network first, ping from application device to gateway server to see if application can access gateway server or not (if there is load balancer in from of gateway cluster, then ping the LB)
for http connection, the default port is 8080, the application should send the request to url, http:<gateway>:8080/<service uri>
you can enable trace on the service, (right click on the service -> service properties -> enable policy debug tracing), send a test request, and then open audit event viewer, if you cannot see audit of the service, that means the request cannot reach the gateway server, should be blocked some where in the middle.
What's the error on application side? connection timeout/ connection refused/ etc. ?