Giant and his Hitmonlee intended to compete in the P1 Grand Prix. However, Team Rocket wanted to compete in order to get the reward. So they stole Giant's Hitmonlee and his clothes before locking him in a bathroom.
Hitmonlee is Giant's only known Pokémon. Giant and Hitmonlee were planning on entering the P1 Grand Prix until Team Rocket spotted Hitmonlee. They took Giant's clothing, Hitmonlee and locked Giant in a bathroom so they could enter the tournament to win the prize.