There’s a difference between a movie that made a lot of money and a blockbuster. Blockbusters make big money fast. When GWTW came out you actually couldn’t have a blockbuster, they didn’t make enough prints, movies wended their way around the country. Jaws was one of the first big steps to the wide release model we have today (though even it went out “phased” with some cities getting it one week and then expanding over the next month). Also Jaws was the first big summer hit, prior to then summer was a dump zone because Hollywood figured nobody would want to see movies during the nice weather. The modern summer blockbuster tent-pole system exists because of Jaws and Star Wars (which proved Jaws wasn’t a fluke).
So yes, Jaws is the first blockbuster. Even with GWTW making more adjusted money.
I checked out the experts at Wikipedia to see what how it was defined:
“Before Jaws set box office records in the summer of 1975, successful films, such as Quo Vadis, The Ten Commandments, Gone With the Wind, and Ben-Hur, were called blockbusters based purely on the amount of money earned at the box office. Jaws is regarded as the first film of New Hollywood’s “blockbuster era” with its current meaning, implying a film genre.[4] It also consolidated the “summer blockbuster” trend, through which major film studios and distributors planned their entire annual marketing strategy around a big release by July 4.[5]”
So evidently the two definitions are separated by “Old Hollywood” and “New Hollywood” demarcation. New Hollywood differs in that it relies on carnival type hype.