Twentieth-Century Realism
When literary folks talk about Realism, they're usually talking about stuff from the nineteenth and early twentieth centuries that gives you an up-close and personal look at the lives of people who are struggling to survive—like the poor, the down and out, and the working class. The only thing, though, is that this stuff was usually written by people who were rich enough to afford being able to sit on their duffs and write novels instead of being forced to shovel pig guts on a factory floor.
A Gathering of Old Men definitely takes us into the world of people who have been oppressed, abused, and marginalized, but the key difference is that Gaines has first-hand experience of what life like that is like—and he's worked that into one seriously powerful novel. Not only that, but Gaines is doing everything he can to let these folks speak for themselves—which is why we hear from them and not some preachy third-person narrator like you might find in a Charles Dickens novel.
Still, A Gathering of Old Men is a novel that tries to paint an accurate picture of the way life is for a very specific group of people in a very specific part of the US, and Gaines doesn't pull any punches. That's why, when it comes to this novel's genre, it's realism all the way.