This is a simple little analog (CV) sync adapter for use with LSDJ and Nanoloop on a Nintendo Gameboy. The 3.5mm jack can be used as either an input or an output, selectable with the on board switch.
This uses a GBA/GBC style link port. To use with an original brick Gameboy (DMG), you’ll need a good dual ended link cable. I’ve used this one and can recommend it: http://www.ebay.com/itm/400690419424