These are ROM hacks built on the Pokémon FireRed or Ruby engine. They painstakingly recreate the Johto region, the story of HeartGold, the Pokémon following system (a hallmark of HGSS), and even the Pokéathlon using GBA graphics and sound.
: Get the .ips or .ups file from community forums like PokeCommunity. pokemon heart gold gba rom
The Myth of the Pokémon HeartGold GBA ROM: Fact vs. Fiction These are ROM hacks built on the Pokémon