I think their connection is a little bit stronger than just one gun purchase. In the documentary "confessions of a mafia hitman" there is a reprsentative from the NJ Organized Crime Task Force who talks about his connection with Demeo(there is even some home footage of Demeo). Also he agreed to plead guilty to and received a life sentence for the murder of Peter Calabro, a bad cop who gave information to the Gambinos. This murder was orderd by Gravano. I find Kuklinski believable in this specif documentary (the later one where he says he killed 200 people is absurd). He talks about all the murders he was convicted of, he talks about killing a random person because Demeo asked him too, and he even talks about Demeos murder. He just says Demeo outlived his usefullness and people thought he was going to run to the cops but doesnt say he murdered Demeo. He says he didnt really care the Demeo was killed and it "couldnt have happened to a better guy" (thick sarcasm). I don't think he was involved in the murders of Hoffa, Galante, Billoti,Favra or Demeo as the book says. And I think a lot of the murders detailed in the book are complete fiction. But after doing some research I do think Kuklinski filled a few contracts with Gravano and Demeo. Not saying he was a part of the crew, or even an important hitman, just that he did a few jobs they didn't want to do, like the killing of a cop (Peter Calabro)