It was a much better gun, despite the mere mm increase in barrel diamter. I suspect the modest diameter increase simplified field upgrades for existing inventory?
Being an anti-aircraft gun, the 76.2mm had a larger propelling charge than the modified French 75mm gun that equipped the M-3 Grant/Lee and the early model M-4 Shermans.
The performance characteristics of both guns are explained in this Wikipedia article: https://en.wikipedia.org/wiki/76_mm_gun_M1
And here is a good article on the various M-4 Sherman tank models: https://en.wikipedia.org/wiki/M4_Sherman
During the war the Army’s 90mm anti-aircraft gun was also modified for use as an anti-tank gun first used on the M36 tank destroyer and, with modifications, I believe was used as the main armament on the M-26 Pershing and later M-46 Patton (Korean War) tanks.
Also buried in the article is a statement that the Army named it a 76.2mm gun, so as to avoid confusion when requesting ammunition with that provided for the 3 inch anti-tank gun, whose shell was 76.2mm in diameter.