There is a significant difference between the English and Japanese releases, namely the cards released in those sets. Gym 1 focused solely on the first four Kanto Gym Leaders (Brock, Misty, Lt. Surge, and Erika). Because of this, there were no Psychic-type Pokémon in the set, and very few Fire-type Pokémon. Wizards of the Coast, when releasing the English translations, mixed in some cards from Gym 2 to increase the type coverage.
Several of the Pokémon belonging to the Gym Leaders were based on the Pokémon they owned in the anime, most specifically Brock and Misty.
A number of cards were exclusive to Theme Decks in Japan. These "fixed" cards are inherently rarer than those released through the main set due to the lack of a rarity symbol. With the release of the English set, Wizards decided to give a number of them a Rare rarity. However, many players felt that a number of these cards, such as Misty's Tentacool, Misty's Goldeen and Brock's Zubat, were not given appropriate rarities, considering the simplistic abilities those cards had. These "common rares" created quite a stir in the Pokémon TCG community. By Gym Challenge however, Wizards decided to base the rarity of fixed cards on their game play abilities over their "fixed rarity" properties.
As the set symbol for Gym 1 and 2 in Japan were the same, new symbols were created for Gym Heroes and the next set Gym Challenge, allowing players and collectors to distinguish between the two. This was the first example of English cards featuring completely different Expansion symbols from their Japanese counterparts, rather than a sized-up version.
Gym 1 was the first expansion in Japan to feature pre-constructed Theme Decks.
Blaine's Moltres, the first card that required 5 Energy cards for an attack, debuted in Gym Heroes (Gym 2 in Japan).
This was the first Expansion released since Base Set that was not a reprint and contained the six basic Energy cards.
This was the first set to feature some given Pokémon on two different cards. While some Pokémon also appeared in two cards in Jungle, Fossil and Team Rocket sets, this was only due to Wizards of the Coast producing non-holographic versions of holographic cards in the set, all other card features being equal. In this set, some Pokémon were appeared in truly different cards, meaning different artwork, attacks, etc. This would become a recurrent trait of future sets.