Why the heck are you so dag obsessed about this? Do you want to see all that junk over again? Do I need to direct you to the archive on
pokemon.com? If you just want satisfaction, Johto already proved he can do all the gyms, and every region since then has done the same. Do I need to direct you to that, too? It has to be Kanto? He already proved that he was worthy of the league anyway. That is your point? You just want the League win? You won't get it from the anime, because Shudo decreed it can't happen, and now that he is dead, that shit is SACRED, they WON'T break it. Even if they did, it won't be what you want it to be. They already had their Misty and Brock feature, and the League is no place to do anything meaningful with them anyway. I'm not sure if there are enough forgotten promises to rematch certain trainers to fill all the League slots, someone will have to do that analysis for me. Even if they do, battles are not the same as they used to be. You want nonsense, they don't write that way anymore. There is no reason for Gary to be there, he retired, and Ash already won against him once. Those would all be the ONLY reason for putting all of this together, and it either can't be done, or there is no reason to do it. You want to be absolved? I will absolve you.
tl;dr pic related: Ash won a League. You can go home now.