iRule to insert route and use gateway

I don't see why you would need an irule for that. Why don't you simply add a static route, destined for your aws server and pointing to your firewall? Also, don't forget to configure some kind of SNAT in order to make return traffic from your aws server arrive on the BIGIP.

Ps: I don't understand how the health checks towards the aws server is working without routing entries. Do you have an explanation for that?