Ash in Indigo, as a trainer seeking to compete in a Pokemon League Tournament, was... He wasn't really good. He tended to rely on luck and fortuitous circumstances (which isn't bad in and of itself, mind you, but comes off ill in consort with his other flaws as a competitive trainer and person during that time frame). He wasn't really invested in bettering his Pokemon's battle prowess and skills, coasting on their innate, baseline capabilities. And Ash's lack of diligence in Indigo is made clear, too. What Ash excelled at over Gary was his more empathetic bond and approach to being with and interacting with most of his Pokemon. (And even then, Ash wasn't at his peak in that regard during Indigo.)
Even if Gary was full of himself in Indigo (which was another of Indigo!Ash's flaws, too), he was still a better League competition Pokemon Trainer than Ash was during Indigo.
That said, I can concur that the louder segments of the Pokemon Anime fandom's tendencies to overplay the Ash and Gary rivalry is a thing. They also overplay Paul, too, albeit for different reasons, but Paul is another can of worms as far as I'm concerned. One I don't feel like opening.