Don't get me wrong, I like Gary. However, Gary and Umbreon shouldn't be able to squat against Ash and Pikachu.Since Ash already did battle both Mewtwo and Lugia, the only thing I can think that could give some conclusion is battle against Gary again and have pikachu finally beating umbreon.
If anything, they’re probably holding Gary's question to the last episodes.
There's no conclusion needed. It's an established fact Ash is leagues ahead of Gary at this point in terms of battling.
Considering how much of a beast Leon was compared to the remaining Master Class contestants and Ash's Pikachu's performance against him; it should alone be able to effortlessly destroy 6 of Gary's pokemon.