Pepijn de Vos created this method to trade and battle Pokemon over the Internet:

TCPoke

a collection of projects that allow you to connect your Game Boy to the internet and trade or battle with the first and second generation Pokemon games

KiCad design files of Teensy 2.0 shield for a Game Link Cable:

Arduino sketch runs on Teensy 2.0 and talks to a Game Boy via raw HID mode:

TCPoke shield kit is sold on Tindie: