Difficult to say without more info.
I had a GP, think I sold it for around 1200 -1300 pounds a couple of years ago. It was in a custom GP GC stock, blue and grey as I bought it S/H action only. Although it looked like a standard one Gary made it for me with two cheek pieces (flat and scaloped). It was also an ex Daystate team gun which was verified to me whilst at the factory getting it serviced.
The serial number is quite important with the GP and service history, later is better if it's been looked after. Can't remember how much a service and battery cost me now I think it was around the £150 mark but that was quite a while ago now.
Steyr Challenge HFT - HW97K - BSA Mercury Challenger - Anschutz 9015 One - AA Pro Target - AA Pro Elite - ASI Paratrooper (R) - Walther LP500