What are credentials? The guy had the credentials to be the best in the world for years and arguably at his best he was the best ever. He beat Segura, Riggs, Sedgman and a young Pancho Gonzalez (who still was a superb player but not quite THE Gonzalez) on tour and beat them easily. And he won a number of Pro Majors. He won three classic majors and that was with an interruption due to World War II. Now to be fair I don't think he was the GOAT but I think he does have a case for it. I think Kramer is superior to players like Roy Emerson, Agassi, Ashe, Nastase, Smith, Vilas, Roche and he has a case over any player including Laver, Federer, Connors, Tilden Gonzalez etc. He is one of the few you can argue to be the GOAT without it being a stupid argument.
For me credentials for the GOAT really have to be eye popping, spectacular achievementsthat go above and beyond, that really show man this dude is on a different level, incredible (stats, records, longevity, strength of play against the field). It's no secret that I think Fed is the best ever (The GOAT) but I think guys like Laver, Sampras, Gonzales, Tilden (though I'm just being respectful here, deep down inside in my heart I can never call a 1920s player GOAT) are in GOAT convo. Guys like Hoad and Kramer I respect what they did but cannot seriously consider them in that category at all. With guys like that, I haven't seen them play, got no highlight reels,have lesser achievements (right I know I can't just go by the fact they have less majors but a lot of other factors and hypotheticals go in place here for pre open era legends that make things complicated). There's also a lot of lore and mythology that surrounds guys like Hoad or Kramer (peak level of play and all that which can be said about anyone who has ever been tough at his peak).
As far I am concerned, these guys are at best on the level of McEnroe or Djokovic. Far away from from the first tier. And I like to keep that first tier (the GOAT contenders) really strict and exclusive to get into. So lots of respect, but no GOAT material imo.