Completely false statement. I have discussed this issue with much older people who saw them both playing live and they choose Rosewall based on the fact that Rosewall won 23 Majors (15 Pro Slams + 8 Grand Slams) while Gonzales "only" won 14 Majors (12 Pro Slams + 2 Grand Slams). 23 > 14. It is not "simplistic" to consider Pro Slams as Majors because they were indeed recognized as Majors those days. It is historically inaccurate to say that they are not Majors.Rosewall didn't become number one until Pancho retired at 32. No doubt in my mind Pancho was the greater player, pretty much all contempories agree as well - not to say Rosewall wasn't very great as well.
"Pro Slams" was a contentious topic in the former pro section for a number of years, what I'll say is that you can't judge pre-open era players by the same standards of today e.g. slam count. For one Pancho turned pro in 1950 and there was no French Pro held until 1955, also the Wembley Pro (which was the biggest of the Pro Majors) wasn't held in 1954 or 1955, Pancho won the event from 1950-1952, made the final in 1953 and won it again in 1956. From 1958 onwards his attendence at the traditional Pro Major events was spotty even in years he wasn't technically retired. He only played 1/6 in 1959-1960, can you imagine a number one player skipped that many slams out of choice? Or even a number one player maintaining their ranking despite that? It was a different world in those years.
Touching on this there was something of a duopoly on the pro tour in Gonzalez's era, there was a tournament circuit including the big pro events such as Wembley and the French and there was the World Pro Tour, which was a series of H2H matches either one on one or as a group, this often involved 100 or so matches and was billed as 'for the world championship' e.g. the number one ranking. It's very important not to ignore the importance of the tours as they were a huge part of the pro life in those years - Gonzalez excelled here and won many (including over Rosewall).
On wikipedia you'll see the US Pro, French and Wembley down as the pro majors but even this is rather simplistic, IIRC the US Pro changed venues and often didn't have the lustre of the other two. There were also less regularly held tournaments such as the Tournament of Champions which Gonzalez won multiple times which was absolutely a major (Rosewall himself names it in an interview as one of the big ones) which isn't counted on his wiki.
As far as actual slams go I barely even count Amateur Slams as they were missing 90% of the best players and the draws were populated by amateurs - Gonzalez was ancient by the time the Open Era rolled around so he can hardly be blamed for not winning more slams then either.
By the way, in 2012 the sports telivision program "100 Greatests of all time" asked an international panel of tennis experts who are the greatest tennis players ever. Rosewall (#20) was ranked MUCH higher than Gonzales (#35).