Location: Route 5

You first encounter Gladion in Route 5 where you encounter him battling with Hau. He managed to defeat Hau and doesn't understand why Hau isn't taking it seriously and is instead finding his loss enjoyable. He then challenges you

Team Skull Gladion	Zorua	Type: Null	Zubat
Battle Type Items	Level 17	Level 18	Level 17
	Attacks: Feint Attack Scratch	Attacks: Tackle Pursuit	Attacks: Bite Wing Attack
	Hold Item: No Item	Hold Item: No Item	Hold Item: No Item

Defeat him and he will explain how he has been doing odd jobs for Team Skull. Then, Team Skull appear and chastise Gladion for stopping them from stealing the local Totem Pokémon and how despite Guzma liking him, he will never be a part of Team Skull. He will then leave.

Location: Royal Avenue

Your next encounter with Gladion is in the Battle Royal Dome in Royal Avenue. He originally goes there to just bury his sorrow in the crowd, but when The Masked Royal spots you and Hau there, he offers a Battle Royal and then invites Gladion to be the fourth person

Team Skull Gladion	Type: Null
Battle Type Items	Level 20
	Attacks: Tackle Pursuit
	Hold Item: No Item

Location: Route 8

Your next encounter with Gladion is a rather abrupt one. In the Motel in Route 8, you will learn that the person renting the room has paid up for a long time in advance. Go inside and you'll find that it is Gladion. Speak to him and he will simply utter his request for you to leave.

Location: Route 13

When you reach Route 13, Gladion will appear in front of you and Hau again, this time mentioning that Team Skull is looking for a Pokémon called Cosmog. Asking if you know anything about it, Hau acts coy causing Gladion to realise you do know about it. He explains how Cosmog may not be powerful but it can bring powerful Pokémon to Alola which would cause disaster and begs for you to protect Cosmog above all else. He then leaves.

Location: Aether House

After you have defeated Guzma in the Shady House, when you return to the Aether House in Route 15 to find Lillie has been kidnapped. He overhears you mention that Lillie took Cosmog and is angry that you and Hau allowed for her and Cosmog to be captured and challenges you to a battle.

Team Skull Gladion	Golbat	Zoroark	Type: Null
Battle Type Items	Level 42	Level 42	Level 43
	Attacks: Acrobatics Poison Fang Venoshock	Attacks: Snarl Hyper Voice	Attacks: Pursuit Crush Claw X-Scissor
	Hold Item: No Item	Hold Item: No Item	Hold Item: No Item

After defeating him, he will apologise for how he acted and mention that you have to go to the Aether Foundation with him in order to save Lillie. Go to the ferry dock in Malie City and he'll be waiting for you and Hau.

Location: Aether Paradise

In the Aether Paradise, you, Gladion & Hau battle through the various Aether employees in a bid to rescue Lillie. This requires you to go to the secret laboratories in the basement of the building. However, the lift won't go down so you need to go up where you battle with Faba, with Gladion by your side. Soon after that, Wicke appears and indicates that she has history with Gladion, calling him master. Going down to the basement, he asks you not to enter the Secret Laboratory A, but to go to Secret Laboratory B. Return upwards and you'll be ambushed yet again, this time where you have a Multi Battle with Gladion

Team Skull Gladion	Type: Null	Golbat	Zoroark
Battle Type Items	Level 44	Level 43	Level 43
	Attacks: Pursuit Crush Claw X-Scissor Iron Head	Attacks: Acrobatics Poison Fang Venoshock Steel Wing	Attacks: Snarl Hyper Voice Grass Knot
	Hold Item: No Item	Hold Item: No Item	Hold Item: No Item

Location: Aether Paradise

Fighting through the rest of the Aether Foundation, you will come topside where Gladion will see a line of Team Skull members and barge through them, rushing to Guzma in order to challenge him, but he loses and starts feeling down that all his training and effort was for nothing, with Guzma condescending him in the process.

After you defeat Guzma, he requests you save Lillie and you go in to stop Lusamine from going to Ultra Megalopolis to stop Necrozma. During this, Lusamine reveals that she is Gladion & Lillie's mother and how disappointed she is in them for stealing her Pokémon, Type: Null and Lillie. Then, forcing Cosmog to create a wormhole, her plan comes to fruition, opening Ultra Wormholes throughout the region.

After that situation, Gladion thanks you for your help and gives you the Moon Flute/Sun Flute in order to go find Solgaleo/Lunala to bring his mother home. He also gives you the Master Ball. He then takes you and Lillie to Poni Island and suggests looking for the Kahuna to get their assistance.

Location: Mount Lanakila

After stopping the situation with Team Skull & Lusamine, when you go to take on the Pokémon League, Gladion will appear, thanks you for what you did, and challenges you to a battle to test his strength and understand why Lillie calls you the best trainer in the world. His Type: Null will have evolved and will be of the type that is super effective against your Starter Pokémon.

Pokémon Trainer Gladion	Crobat	Zoroark	Silvally	Lucario
Battle Type Items	Level 53	Level 53	Level 55	Level 53
	Attacks: Acrobatics Cross Poison	Attacks: Night Daze Hyper Voice Grass Knot	Attacks: Crunch Crush Claw X-Scissor Multi-Attack	Attacks: Aura Sphere Metal Claw Quick Attack
	Hold Item: No Item	Hold Item: Darkinium Z	Hold Item: Fire Memory	Hold Item: No Item
Rowlet Chosen

Pokémon Trainer Gladion	Crobat	Zoroark	Silvally	Lucario
Battle Type Items	Level 53	Level 53	Level 55	Level 55
	Attacks: Acrobatics Cross Poison	Attacks: Night Daze Hyper Voice Grass Knot	Attacks: Crunch Crush Claw X-Scissor Multi-Attack	Attacks: Aura Sphere Metal Claw Quick Attack
	Hold Item: No Item	Hold Item: Darkinium Z	Hold Item: Water Memory	Hold Item: No Item
Litten Chosen

Pokémon Trainer Gladion	Crobat	Zoroark	Silvally	Lucario
Battle Type Items	Level 53	Level 53	Level 55	Level 53
	Attacks: Acrobatics Cross Poison	Attacks: Night Daze Hyper Voice Grass Knot	Attacks: Crunch Crush Claw X-Scissor Multi-Attack	Attacks: Aura Sphere Metal Claw Quick Attack
	Hold Item: No Item	Hold Item: Darkinium Z	Hold Item: Grass Memory	Hold Item: No Item
Popplio Chosen

During the game's credits, we will see Galdion get on a boat. If you speak to Wicke in the Aether Foundation, she will note that Gladion has gone to Kanto and Johto in order to be a strong trainer. She will also give you the Type Null that he was saving for you

Type: Null Level 60

OT: Yours ID: Yours Ability: Battle Armor Hold Item: None

??? Nature. Date of Receiving Ancient Poni Path. Caught at Lv. 60

Metal Sound Iron Head Double Hit Air Slash

Challenger: Gladion
Specialty: None

Details:
Having left Alola to go train in Kanto and Johto, on rare occasions, Gladion will return to the Pokémon League to put forward what he learned against you. His team has an advantage over yours and he now has Kanto Starters.

Pokémon Trainer Gladion	Crobat	Zoroark	Porygon-Z	Lucario	Venusaur	Silvally
Battle Type Items	Level 68	Level 68	Level 68	Level 68	Level 68	Level 70
	Attacks: Acrobatics Cross Poison X-Scissor Steel Wing	Attacks: Night Daze Hyper Voice Grass Knot Foul Play	Attacks: Tri Attack Dark Pulse Ice Beam Thunderbolt	Attacks: Aura Sphere Flash Cannon Extreme Speed Psychic	Attacks: Petal Blizzard Sludge Bomb Earthquake Outrage	Attacks: Crunch Crush Claw X-Scissor Multi-Attack
	Hold Item: No Item	Hold Item: Darkinium Z	Hold Item: No Item	Hold Item: No Item	Hold Item: No Item	Hold Item: Fire Memory

Pokémon Trainer Gladion	Crobat	Zoroark	Porygon-Z	Lucario	Charizard	Silvally
Battle Type Items	Level 68	Level 68	Level 68	Level 68	Level 68	Level 70
	Attacks: Acrobatics Cross Poison X-Scissor Steel Wing	Attacks: Night Daze Hyper Voice Grass Knot Foul Play	Attacks: Tri Attack Dark Pulse Ice Beam Thunderbolt	Attacks: Aura Sphere Flash Cannon Extreme Speed Psychic	Attacks: Air Slash Flamethrower Earthquake Outrage	Attacks: Crunch Crush Claw X-Scissor Multi-Attack
	Hold Item: No Item	Hold Item: Darkinium Z	Hold Item: No Item	Hold Item: No Item	Hold Item: No Item	Hold Item: Water Memory

Pokémon Trainer Gladion	Crobat	Zoroark	Porygon-Z	Lucario	Blastoise	Silvally
Battle Type Items	Level 68	Level 68	Level 68	Level 68	Level 68	Level 70
	Attacks: Acrobatics Cross Poison X-Scissor Steel Wing	Attacks: Night Daze Hyper Voice Grass Knot Foul Play	Attacks: Tri Attack Dark Pulse Ice Beam Thunderbolt	Attacks: Aura Sphere Flash Cannon Extreme Speed Psychic	Attacks: Surf Ice Beam Earthquake Outrage	Attacks: Crunch Crush Claw X-Scissor Multi-Attack
	Hold Item: No Item	Hold Item: Darkinium Z	Hold Item: No Item	Hold Item: No Item	Hold Item: No Item	Hold Item: Grass Memory