From what I understand it originally referred to a specific fetish, but it got stolen and is now just the new way to call someone something akin to "porn addict", "degenerate" or anything else to that effect.
To talk about the original fetish though, from what I understand it's a response to all the "porn addiction" hysteria. Basically, the idea that consuming this content can turn you into some sort of brain-dead pleasure drone who only wants to keep consuming. Essentially, Gooners (the fetishists themselves) are a bunch of people who think that it would be really hot if porn addiction was a real thing. I've seen it compared to hypno, except instead of another person it's the content itself that is doing the hypnosis. The whole idea of getting a bunch of monitors and having porn on all of them, or hiding away in a "goon cave" isn't actually about the porn itself. It's about the idea of it, the idea of having your brain turned to mush as you melt into being a hedonistic drone concerned only with your own pleasure, enslaved by the fake world on the other side of the screen, able only to make yourself feel good.
I remembered seeing this image floating around here a while ago because people found it amusing, but I went looking for it again because I think it's a good example of the whole phenomenon:
I myself am not a gooner. I am not a primary source on this matter, so take what I have to say with a grain of salt. But I do have respect for them, to see a whole ass moral panic and then to decide to touch your junk about it shows the unstoppable drive of humanity. That you can never keep a horny person down. It's counter cultural and subversive, a celebration of what is scorned that does not try to make itself acceptable, but instead leverages its unacceptability to push into new territory. Gooners are majestic animals and they have my respect.