The Gym Heroes set was the fourth first generation expansion. Following on with the new Pokémon concepts, this set introduced Owner Pokémon. These Pokémon are owned by various trainers, in this case the Kanto Gym Leaders. Gym Heroes focuses on Brock, Misty, Lt. Surge & Erika, but also contains some cards owned by Sabrina, Blaine & Rocket. It was an unusual set in that it had many Pokémon with rarities of Rare for Pokémon cards that people would generally feel should be Common. This is due to them originally being only in special theme decks in Japan.
It is also the first set since Base to include Basic Energy Cards. It also introduced Stadium Cards which get placed into play and give new effects to the gameplay.