Successful stand-up comedians are natural subjects for documentaries, because they carry an inherent tension. Many are loners addicted to attention (as well as other substances), and their larger-than-life personalities can be the stuff of great, revealing films. Then there are outlier comedians like Gilbert Gottfriend, the subject of Neil Berkeley’s documentary. He’s the comedian’s antithesis: a genuine, self-effacing man without a visible drug or alcohol problem to speak of. And this makes Gilbert, the documentary, a crashing bore.