Kanto's gym leaders are among the most adept Pokémon trainers. Masters of their craft, even Pokémon gym leaders have similarities to the witches and wizards of Harry Potter. Age is not a factor because even adults can have a Hogwarts House. The whimsical Sorting Hat of Harry Potter determines which House a student is sorted into. Their newly found identity carries on into adulthood.
10 Brock - Gryffindor
To kick things off, let's start with the first gym master you battle in the game. In the Pokémon anime, Brock is one of the show's main characters. It is a no-brainer to sort Brock into the Gryffindor House. The trio of Ash, Brock, and Misty (two boys and a girl) seems all too familiar.
Gryffindor members are said to be courageous, daring, and sometimes reckless. Brock shares all these traits with his companions in the anime. He's an oddball for getting googly-eyed every time he sees a pretty girl, but a Gryffindor nonetheless.
9 Misty - Gryffindor
In the Pokémon games, Misty in the second gym leader you must face. She puts up a heck of a battle for players who aren't prepared. Misty gets sorted into Gryffindor for the same reasons as Brock.
She is courageous and headstrong but is also the voice of reason between the trio of protagonists. Misty can be temperamental at times, just like Hermione. Her specialty is the Water-type, which contrasts from Gryffindor's element of fire, but the Sorting Hat works in mysterious ways.
8 Lt. Surge - Hufflepuff
Lt. Surge is the macho gym leader from Vermilion City who specializes in Electric-type Pokémon. Also known as the "Lightning Lieutenant," Lt. Surge was an American soldier before becoming a gym leader.
Given his track record, the Sorting Hat would sort Lt. Surge into Hufflepuff. Hufflepuff members are known for being hard-working and loyal, but usually fair. They can be seen as overly strict, but deep down, they are nice people. Lieutenant Surge is all of these things, and there isn't a place he would fit in better.
7 Erika - Ravenclaw
As someone who has the biggest garden in Celadon City (and maybe Kanto), Erika is well-known for her creativity. As someone who teaches flower-arranging, Erika would be a shoo-in for Ravenclaw.
She'd fit right in with other Ravenclaws like Cho Chang and Lisa Turpin. Erika is a trainer known for her Grass-type Pokémon and for being someone who admires the beauty in things. In the Harry Potter universe, she would, without a doubt, be a fastidious Ravenclaw.
6 Koga - Hufflepuff
As someone who is a student of ninjutsu, an advanced form of martial arts known for guerilla tactics and unconventional fighting moves, Koga displays great discipline. Given Koga's ninja experience, he displays dedication beyond that of a normal gym leader. For the aforementioned reasons, Koga would be a sublime fit for Hufflepuff.
Hufflepuff members are hard-working and patient. As a trainer who specializes in Poison, he enjoys inflicting pain. It wouldn't be out of the question to place Koga in Slytherin. It's a close call, but Koga would be a devoted Hufflepuff.
5 Janine - Hufflepuff
For those players who played the second generation of the Pokémon games (Gold, Silver, and Crystal), you may remember Janine. She is the daughter of Koga, who runs the gym in the absence of her father. Little is known about Janine in comparison to Kanto's other gym leaders.
Like her father, she is a ninja. Janine changes the appearance of the gym by removing its invisible walls and by disguising her assistants. We can assume that the Sorting Hat would place Janine into Hufflepuff. It wouldn't be the first time the Sorting Hat placed family members in the same Hogwarts House.
4 Sabrina - Slytherin
Sabrina plays the antagonist in the Pokémon anime, but it's abundantly clear that the Pokémon games are separate from the anime. As reported by thegamer.com, "Sabrina wields a whip during her Gym battle against Red."
This might mean that she whips her Pokémon, but why would someone do that if they were a serene trainer? Furthermore, Sabrina's psychic powers give us the impression that she is evil. She can see into the future as if she were looking into the Mirror of Erised. The Sorting Hat would sort Sabrina into Slytherin.
3 Blaine - Ravenclaw
Blaine shows intelligence above that of other gym leaders. In the Pokémon games, players have to take a lengthy quiz before battling Blaine. He punishes you for answering questions wrong. Blaine is more likely to be a professor at the Hogwarts School of Witchcraft and Wizardry than a student.
As someone who fights with the Fire-type, Blaine would be a good fit for Gryffindor. However, someone as brainy as Blaine would be better suited to Ravenclaw. Unlike Ravenclaw Professor Gilderoy Lockhart, Blaine is smart and not a fraud.
2 Giovanni - Slytherin
Aside from speaking a peculiar language of snakes, Giovanni fits the description of a Slytherin. His gaudy outfit, combined with his swanky mannerisms, makes him the ideal candidate for the Slytherin House.
As someone who leads Team Rocket, the darkest organization in Pokémon, there are some disturbing parallels between Lord Voldermort and Giovanni. They are both the same age, plan to take over the world, and wouldn't be against stealing candy from a baby. Godric Gryffindor and Albus Dumbledore would be appalled by Giovanni's disturbing background.
1 Blue - Slytherin
As someone who rivals the protagonist and has a snarky attitude, Blue would be a surefire success in Slytherin. During the Sorting Hat ceremony, Blue would be like Draco Malfoy because the Sorting Hat would choose Slytherin before touching Blue's head.
It can also be argued that Blue is a good character who would match with Gryffindor. The Sorting Hat often mistakes Gryffindors for Slytherins. It's a close call, but for his rivalry with the protagonist, he is a Slytherin.