This has got to be one of the coolest DIY mods I’ve ever seen. What you’re seeing that picture is an actual Gameboy device gutted and modded to act as a game controller for a Galaxy Nexus. Now before you go and destroy your old Gameboy trying to replicate this make sure you know what you’re doing.
This mod does require some know how and a few of additional pieces. First, the screen was removed and a Galaxy Nexus case was put in its place. Then a Wiimote was taken apart as it’s able to pair with the phone via Bluetooth. That was then stuffed inside and buttons needed to be rerouted to work with the buttons on the Gameboy itself. Add to the part an app called “Wii Controller IME” and you got yourself a fully functional Gameboy dock to play all your old favourites via an emulator.
Again, while it all looks and sounds relatively easy I wouldn’t recommend jumping into this unless you really know what you’re doing. I take no responsibility for you destroying your old Nintendo relics!