Setting up a torified ssh tunnel to a squid proxy

The tunnel?

And in the gateway, for this type of setup, most likely a strong no.

If you setup an ssh tunnel from Whonix-Gateway to the VPS, you actually de-anonymize yourself. Setting up a ssh tunnel in Whonix-Gateway results in user → ssh → Tor → Internet. Some people want this to have a longer tunnel length but in your case that would be a huge fail.

See:

It’s Tor not TOR.

Always prefer Tor Browser over icewasel. (Why? → Tor Browser Essentials)

I recommend strongly against X11 forwarding through ssh due to keystroke fingerprinting. (At risk as soon as the VPS turns malicious.)

Also applicable to you:
Remote Administration - Whonix