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:

tcpoke_shield

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

tcpoke_teensy

TCPoke shield kit is sold on Tindie:

Hackaday reports on this mix of Pokemon Go and DIY electronics:Beware Of Tall Grass: Pokemon Go on the Gameboy Pocket

[Pepijn de Vos] was excited to interact with the world’s most popular augmented reality pedometer, Pokemon Go, and was extremely disappointed to find that his Blackberry couldn’t run it. Still, as far as he could tell from behind his wall of obsolete technology, Pokemon Go is all about walking distractedly, being suspicious, and occasionally catching […]

TCPoke shield kit on Tindie: