I believe this is the only episode in which Gomer and Goober appeared together. They appeared together, in one episode, in the Gomer Pyle, USMC TV show.
Gomer Pyle USMC RULED. Goober's episode was one of the weaker ones though. I never liked The Andy Griffith character or show. I thought he was incredibly patronizing and condescending to Barney Fife. But he was a good "dad" to Opie.
BTW, I had no idea that Andy and Barney were cousins.