Moved this to development sub forum. If this was a user support question, I would just say ‘unsupported’. ( https://www.whonix.org/wiki/FAQ#What_do_you_mean_by_unsupported.3F )
This requires quite some thought and whonix-gw-firewall modifications. I am personally a bit more interested to have a VPNBOX (using a VPN as full replacement for Tor) rather than selective circumvention maintained by someone else, but nevermind.
I suggest step by step:
- Not start with Whonix. Getting a VPN-only-Gateway working first.
- Making that a “selective VPN gateway”.
- Getting Whonix-Workstation to work with the VPN-Gateway.
- Modifying Whonix-Gateway.
Or another approach, take a plain Debian [jessie] - and then review, and add one essential Whonix network related package one by one until you have the most basic functionality without sdwdate and stuff. Perhaps that would help to demystify Whonix generally.
(Btw the Whonix VPN tunnel combination instructions have recently been improved. - Instructions on how to disable uwt and socks proxy settings may be useful.)
(Btw to understand Whonix internals better generally, this page might help. Just skim through it.
Somehow I more and more dislike such pages. Makes it all look more, and more complex and daunting than it actually is.)