A musician from lower Saxony named eVADE/duality micro has produced a cable to sync up Game Boys running popular homebrew software with Korg drum machines and synths.
On the Game Boy side, you need either LSDJ or nanoloop cartridges for music tracking. The cable then connects to the Gamelink Port or your Nintendo handheld.
On the Korg side, the volca series and monotribe can take that input via their sync port. (The Game Boy has to be the clock master with LSDJ, at least, for now; nanoloop appears to work both directions. A future LSDJ may also send.) I believe this should also work with devices like Teenage Engineering’s Pocket Operator line or some iOS apps – anything capable of responding to the Korg-style sync pulse, which has become something of an unintentional de facto standard.