When creating a Standalone Qube based on the whonix-workstation template and setting sys-whonix as its net-vm, the gateway address will be sys-firewall.
It still runs on sys-whonix though, so might be a glitch.
But if you set “no network” when creating the Standalone, and then change the net-vm to “sys-whonix” after creating it, the gateway address will be sys-whonix