It's not clear what IP addresses OP is referring to.
The Whonix gateway has two adapters. One adapter (eth0) may connect to the VirtualBox host, or to a router VM. It will report (using ifconfig) whatever IP address the corresponding DHCP server gives it.
The other adapter of the Whonix gateway (eth1) connects to a VirtualBox internal network (by default, "whonix") which it shares with the workstation. The gateway and workstation (obviously) have different local IP addresses on that internal network. By default, the gateway is 192.168.0.10, and the workstation is 192.168.0.11 (both static).
As adrelanos notes, Whonix uses stream isolation, so each isolated application will have a unique public IP address via Tor. However, because the workstation connects through the gateway, particular applications running on either should report the same IP address at any given time, until the circuit changes.
Firewall and routing rules on the gateway prevent local applications from accessing anything via eth0 except through Tor.