This is a fairly simple ROM hack that adds the female trainer, Green/Leaf from Pokémon Fire Red and Leaf Green into the original Pokémon Red and Blue, made using pret's pokered disassembly. Much like in Fire Red and Leaf Green, Professor Oak will ask the player if they are a boy or a girl when the game begins, and the game will continue as normal from there, including altering all references to the player's gender depending on their decision.
Inside the included zip are two patches: one for Pokémon Red and one for Pokémon Blue, as the two games have a few differences.
Pokemon Red (UE) [S][!].gb
CRC32: 9f7fdd53
MD5: 3d45c1ee9abd5738df46d2bdda8b57dc
SHA-1: ea9bcae617fdf159b045185467ae58b2e4a48b9a
SHA-256: 5ca7ba01642a3b27b0cc0b5349b52792795b62d3ed977e98a09390659af96b7b
Pokemon Blue (UE) [S][!].gb
CRC32: d6da8a1a
MD5: 50927e843568814f7ed45ec4f944bd8b
SHA-1: d7037c83e1ae5b39bde3c30787637ba1d4c48ce2
SHA-256: 2a951313c2640e8c2cb21f25d1db019ae6245d9c7121f754fa61afd7bee6452d