Sebastian Tomczak has posted info on a $1 Nanoloop MIDI Sync for Game Boy.

Overview:

So here is a very simple circuit that you can use to synchronise Nanoloop (for the Nintendo Game Boy) to a MIDI clock source (such as a sequencer or a drum machine).

This circuit works and has been tested, but of course I can’t guarantee that if you build it, it will work. At any rate, at least you won’t be wasting much money because the only component that is needed is the 4017 IC, which cost me about $1.

This schematic does not adhere to MIDI hardware specifications (there is no optocoupler, for instance) and there are no filter or bypass caps for the Game Boy power supply. So these are the sorts of things you might like to add (or not). However, I have it working in a very stable fashion ‘as is’.

One thing to note: you can only send MIDI clock messages on the output that is connected to the Game Boy, because other MIDI messages will mess up the synchronisation. Although some may find this a little limiting, I always tend to use a dedicated port for any MIDI clock sources anyway, so it suits me just fine.