For Qubes + Whonix, are you using the old Dual-HVM platform or the new ProxyVM + AppVM platform?
Port forwarding through your local clearnet indicates that you do not want to run that traffic over Tor, correct?
Otherwise, if wanting to use Tor, doesn't the application's traffic route through Tor normally and then use the app-specific ports at the Tor exit node (and not your local clearnet ports)?
If wanting to run over clearnet (not Tor), then maybe using a non-Whonix standard Linux AppVM would be easier to get setup with clearnet port forwarding.
If wanting to run over Tor, then maybe double check the above notion of needing to port forward locally? Maybe you don't need to for Tor?
I personally haven't gone through the Qubes port forwarding instructions yet.
But the Whonix-Gateway does cut off clearnet connections with its firewall, which can be bypassed, if totally necessary (but not recommended).
Maybe us looking at a link to your application's install steps would help further diagnose.