Gladion will be making his appearance, though he owns a Midnight Lycanroc in the anime:
Some thoughts:
1. Being they already revealed their relation (and them diverging from the game's story), I wonder what Gladion's deal is going to be in the anime? Is he still working for Team Skull? In the pics it looks like he's having a plain conversation with Lillie and his Lycanroc is giving Ash's Rockruff a nice pat on the head (ominous as it looks).
2. Well, guessing Ash is going to evolve Rockruff to Midday, now that we may have his rival for Alola (and Hau doesn't look like he's making an appearance any time soon).
3. Not only does it look like he has Lycanroc in an Ultra Ball, it looks like it's using the Rock-type Z-Move, Continental Crush. So Gladion can use Z-Moves? I don't see his Z-Ring.
4. Why don't Gladion have a Midnight Lycanroc in the games? It's astonishingly how fitting it is for him. The color scheme (and clothing damage going along with Lycanroc's claws), wily hair, attitude, and Gladion's earrings sort of match Lycanroc Midday's "ribs".
5. So Lillie doesn't have Cosmog and Gladion doesn't have Type: Null, where are we going with this story, Pokemon Company? Are you just setting up things for Alola's second season which will start having a story (hopefully matching close to the games)?