I think it was underdeveloped in some areas (definitely agree that Gary needed to at least appear more in Johto), but it's iconic and still largely works for what it is. It was the beginning, so it made sense for the rivalry to be so simple. Ash and Gary were both immature brats at the beginning, but by the end are much less petty. Early Kanto also had a very different tone than every other series, so you kinda have to take certain things with a grain of salt.
I also think Gary's definitely the best way to do an "asshole just for the sake of it" rival. He and Ash were rivals even before starting their journeys, so they can actually get away with how petty the rivalry was at first. Compare that to say, Trip, whose dislike for Ash isn't fueled by anything. Is he just an asshole in general? Is he just immature like Gary was? Even if that's the case, what makes his rivalry with Ash special then? There's no reason for him to dislike Ash more than any other "weak" Trainer. Gary and Paul actually had reasons to especially dislike Ash.