He is the father of Papa Cosmo, the father-in-law of Mama Cosma and the grandfather of Schnozmo and Cosmo Cosma. He is also the great-grandfather of Poof. Little is known about him, for he has never truly appeared in an actual episode.
He resembles the other members of Cosmo's family, but is nearly bald and has only one tooth. It is possible he may have had green hair when he was younger because of his eye color and all of his descendants (besides Poof) has green hair
In "Fairly Family Tree" the video explains that Grandpa Gonzo wore diapers in his late years along with the fact that he drooled.