Walkthrough: After returning to the Aether House from Po Town, you'll quickly find out from Hau and the others that Lillie and Cosmog were kidnapped by Plumeria and other Team Skull Members, not having been strong enough on their own to stop them. As you find out for yourself, Gladion will appear and overhear everything as well. He will become very angry about how Lillie had Cosmog this whole time, and even more so for how you and the others allowed for both of them to be kidnapped. He will then challenge you to a battle.
Team Skull Gladion - Level 42 Golbat, Level 42 Zoroark, Level 43 Type: Null
After defeating him, Gladion will calm down and apologize for how he had acted. He then mentions that you have to go to Aether Paradise to save her, that he knows she'll be there. He will then tell you and Hau to meet him at the ferry in Malie City.