At his best Kramer was fabulous. He defeated Riggs, Segura, Gonzalez and Sedgman fairly easily on tour. Yes I do think he is viable as a GOAT contender especially when you consider how long he was number one.
He didn't have the number of majors that the media and fans tends to fixate on today but it really wasn't as important in those days. He was the top player and that's all there was to it. Many people consider Kramer as the best player of all time including greats like Frank Sedgman.