Isn't this exactly what trash80's mGB is?

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

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

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

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

Considering the gameboy soundchip isnt a REAL sound chip like the ym or sid, youd probably be better off with just making an actual softsynth on a dsp chip.

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?!

Hello, recently I started to play with electronics and Arduino (so I´m a newbie, sorry If my answer is naive). I have gameboy with LSDJ and other soft, but for a long time I have a plan to build HW synth, which will use gameboy CPU unit (or it´s part responsible for sounds) for generating sounds. Something similar – in principle – like for example MIDIbox SID. I´m trying to investigate if this idea is somehow accomplishable... I wonder If it is possible to desolder gameboy CPU and then handle it directly by sending current to its pins. Preferrably by Arduino, but this is my plan for future, so now I only need to know If my idea deserves further effort. From documentation which I found so far I´m not sure, for example i don´t know if DMG CPU can run standalone on my breadboard without other parts from original gameboy and without booting from proper cartridge etc. ... Thank you very much for your opinions and sorry for my english!