I am definitely not a dog expert (as you can see by the problems I'm having with Max), but I think I understand what's going on with Gonzo.

Have you ever had somebody you didn't know very well do something really mean and unexpected to you? For a while, you don't trust anybody. After you've spent some more time with other people, though, you realize that most people aren't going to be mean. You may remain a bit less trusting, in general, but you don't always expect people to hurt you.

Maybe it will just take some more time with dogs who aren't mean to him, before Gonzo realizes that he can be more trusting again.

I would think that some time spent with one or two gentle large dogs at a time might help Gonzo to begin regaining his trust in others. The trouble will be finding these trustworthy but strange dogs to socialize him with.