I think I understand what you're saying, but if it was so vile and so toxic, why would people keep returning to it and stay so involved? Whether you happen to like it or not, these are communities just as much as any other. I think it would be better for the human condition if, instead of dismissing them as "vile toxicity", we looked at what they're saying and what's caused them to feel a need to say it.
Their opinions are plainly wrong by any modern standard (i.e. killing "subhumans", etc.), so there's no merit in approaching them on that front.
Though as you say, it might be necessary to understand them to prevent such extremism from spreading. But at that point it's very well worth asking if all that 'understanding' is really more effective than just banning such platforms.
Worked for Reddit, for instance. There was a study that showed that the prevalence of hate-speech was decreased after they banned the racist subreddits. Even people engaged in those communities became less extreme after you've removed their echo chamber.
It "worked" for Reddit because people moved over to Voat or back to the chans. Just because the community moves off a certain platform doesn't actually change the existence of either the people or their opinions. I want to make clear that I agree with you that an opinion that terms any member of genus Homo as subhuman is wrong; and that killing is also wrong. On the other hand, I for one am curious about how these memes originated and continue to propagate; and I feel that direct engagement is the only way we'll find that out. Massive cultural impact cannot be made by simply avoiding other opinions. After all, Columbus massively underestimated the circumference of the Earth compared to Eratosthenes - but the Portuguese still funded his travels, and his trip was successful, even if by complete accident. Engaging with those who have wrong opinions can sometimes have unexpected positive impact.
Where, pray tell, is the proper place to engage with these people as fellow human beings then in your opinion? Is it loosely defined as a place that the rhetoric and world-view you prefer (and which I probably prefer as well) rules with an iron fist, with people who are actively censoring what someone says and forcing them into changing the way they address you? Over time, lines have been drawn, and many people have become disenfranchised based on the unwritten rules of certain Web communities. I'm not suggesting for a moment that you'll change your mind suddenly and agree with their views - or that I would. The people who feel strongly about certain issues have formed communities, and they supposedly defend the right and virtue of free speech. So, speak to them! Freely. Don't seek to control, but to understand. And then when you do understand, if you can find common ground, work from it. If not, sit back and consider where your morality differs so distinctly from another member of the human race that you can find no core values in common. I've found doing that makes for a much better result in my dealings with both left-wing and right-wing radicals - because I can usually find at least something we both see as positive.
"Where, pray tell, is the proper place to engage with these people as fellow human beings then in your opinion?"
Somewhere not blocked by the anonymity of the internet. When online, especially at a place like Gab, or even Twitter, people are predisposed to behaving in more of a trollish, uncharitable way. In person, many of those behaviors don't happen.
"with people who are actively censoring what someone says"
I'm sorry, but I can't continue after this. You set up this sob story about not treating those on the alt-right as people, yet decry others telling the alt-right that they have to treat others as people, and not, for example, say that Jews should be treated as livestock. Its part of this really shitty mindset that people must tolerate the alt-right, but they don't have to tolerate anyone else, and should be encouraged, even, to shit on others.
If they want respect, they need to show it to others. If they're not willing to do that, then it's no surprise that they feel they get no respect in turn.
