Nikolay,
I don't know of any schematics out there for these units, but I owned a GP11 with a matching power amp for about five years and was never happy with it. It tended to suck the life out of an otherwise really good bass tone. When I was still new to recording, I would allways DI the bass through the GP11 until I realized that it sounded better getting DI'ed straight into my old Mackie mixer.
Is this something that you are wanting to build? The resale value for these units is pretty low. I'm sure that it would be cheeper to pick up a used unit. I've seen them from $150-$200 USD.
Regards,
Chris