Walkthrough: After defeating the final Trial on Poni Island and capturing the Cover/Mascot Legend, you will finally be allowed to take on the first ever Pokemon League of The Alola Region. The Pokemon League is found at the peak of Mount Lanakila, as Professor Kukui first told on the other mountain peak. But as you first approach the base of Lanakila, Gladion will descend from the mountain and challenge you to a Battle first. His team has improved immensely.
Trainers:
Gladion (Rowlet Starter) - Level 52 Crobat, Level 52 Weavile, Level 52 Lucario w/ Steelium Z, Level 53 Silvally w/ Fire Memory
Gladion (Litten Starter) - Level 52 Crobat, Level 52 Weavile, Level 52 Lucario w/ Steelium Z, Level 53 Silvally w/ Water Memory
Gladion (Popplio Starter) - Level 52 Crobat, Level 52 Weavile, Level 52 Lucario w/ Steelium Z, Level 53 Silvally w/ Grass Memory
Afterwards: After defeating Gladion, he will return back to the Aether Paradise to continue to reestablish their purpose, as well as look after his mother. He will acknowledge that though you still aren't the best of friends, you are not a foe either. He will also give you a Max Elixir from Lillie, who he will tell that you are taking on the Pokemon League.