Pokemon GS Chronicles is a Game Boy Advance ROM hack that offers a unique storyline and features within the Pokemon world.  Version 2.7.1 suggests multiple iterations with improvements and additions since its initial release.  This fan-made game, likely drawing inspiration from the original Pokemon Gold and Silver games, is developed outside of Nintendo or Game Freak, using the engine of an original Pokemon release as its foundation.

Pokemon GS Chronicles 2.7.1

Pokemon Fused Green 2.3.2