All traffic from Whonix-Workstation ™ and Whonix-Gateway ™ is routed over Tor.    
Since Whonix 0.2.1, Whonix-Gateway ™ traffic is also routed over Tor. In this way, use of Whonix is hidden from persons or systems observing the network.
To preserve the anonymity of a user's Whonix-Workstation ™ activities, it is not necessary to torify Whonix-Gateway ™ own traffic.
For reader interest: If DNS settings on Whonix-Gateway ™ are changed in
/etc/resolv.conf, this only affects Whonix-Gateway ™ own DNS requests issued by applications using the system's default DNS resolver. By default, no applications issuing network traffic on Whonix-Gateway ™ use the system's default DNS resolver. All applications installed by default on Whonix-Gateway ™ that issue network traffic (apt-get, whonixcheck, sdwdate) are explicitly configured, or forced by uwt wrappers, to use their own Tor
SocksPort (see Stream Isolation).
Whonix-Workstation ™ default applications are configured to use separate Tor
SocksPorts (see Stream Isolation), thereby not using the system's default DNS resolver. Any applications in Whonix-Workstation ™ that are not configured for stream isolation - for example
nslookup - will use the default DNS server configured in Whonix-Workstation ™ (via
/etc/network/interfaces), which is the Whonix-Gateway ™. Those DNS requests are redirected to Tor's DnsPort by Whonix-Gateway ™ firewall. Whonix-Gateway ™
/etc/resolv.conf does not affect Whonix-Workstation ™ DNS requests.