FunnyPlaying has done their own version of the GBC and GBA PCBs you can buy. I don't know of any open source versions.

I found something on nataliethenerd's GitHub, but the license is non-commercial.

https://github.com/nataliethenerd/CGB_ReverseEngineer

>I scanned, sanded and redrew the CGB schematics using a CGB-CPU-04 board. The original schematic was used as a refrence for values.