I bet you are now stealing this idea and from now you are all working secretly on awesome gameboy synths in order to get filthy rich, aren´t you?!

Oh yeah people have been biding their time since 1989 to do this oh yup yup

Siegfried wrote:

What do you mean if you say "REAL"? Thank you for explanation...

The gameboy doesn't have a dedicated, stand alone sound chip. All sounds are generated from the gameboy cpu.

I would add that to use the GB 'PSG' you need to have the GB CPU writing values to the internal registers. Unlike the SID that is a dedicated chip interconnected to the C64 CPU with an external buss on the PCB.

Nintendo used this same integrated design on the Famicom and NES, so you see the midiNES and Chip Maestro projects that control the 2A03 CPU in order to produce sound. These take advantage of the cart port bus interface to exchange data with the 2A03 CPU.

In the SID's case you can recreate the the needed bus signals with any microcontroller to make it produce sound. But for the GB, you would need to have the GB CPU running some code that 'talks' to  an external microcontroller and manages the internal 'PSG' register writes.

So you are better off using/developing software like nanoloop or LSDJ that talks to external controllers via the existing ports. 
Yogi

What do you mean if you say "REAL"? Thank you for explanation...

The gameboy doesn't have a dedicated, stand alone sound chip. All sounds are generated from the gameboy cpu.