The Tor part has to be done outside of gossip, gossip doesn't have any tor-specific code. That includes tor-enabled DNS, if you wanted to use a .onion relay. If it is not working, I don't think it is gossip that is not working.

I don't see any setting to tell gossip to use Tor (or any other proxy), what works for me are netns tricks with veth, bridges and tun2socks and dnsproxy, so all the internet traffic is redirected to Tor by default.