Frankly, it would've been prudent for us to have gotten a hint here about Glion's eventual comeback at the Shinou League, yet Glion was left behind without Satoshi promising to retrieve him later from what I remember.
Ash's Gliscor deciding to stay behind for training was the weakest part of the second half of Sinnoh. It should've kept traveling with Ash instead of leaving to make space for Gible.
I really liked this “goodbye” for Gliscor. Personality-wise Gliscor is my favorite of his Sinnoh Pokémon; so fun as well as just being cool and good in battle as well. Gliscor gaining courage and the will to get stronger and stay to train was executed very nicely. I loved seeing Dawn and Ash help it train to feel the air.
Gliscor being a crybaby but resolving to stay was so cute and nice, and that goodbye at the end def pulled at my heartstrings. Even TR didn’t want to interrupt lol