Voice has to come from an engaged person to be worth listening to, but that doesn't mean that what the voice says has to be about the person. Every great writer without exception has had a voice, but most of them use that voice to show us how the world looks to them. Some few focus on themselves as their subject. And the great exemplar of gonzo writing - Hunter S. Thompson - used himself as a subject far more than traditional journalists did. Even Thompson, however, wrote about his own exploits as a way of exposing truths about the world, whether it was Hell's Angels or standard party politics. The point about engagement and gonzoism isn't that you have to talk about (and thus promote) yourself, but that whenever you do talk, you're honest about the fact that you're writing from a point of view. That's how I understand it, anyway. Example: Benetton writing about death row inmates; it's gonzo because it's engaged even though the author doesn't show up in it.
Having said that, however, if you're writing in a gonzo way, there is obviously a strong temptation to put yourself into the center of your story. I just don't think that's essential to gonzoism.