Is it a fairly modern rifle?
The Airsporter 'S' had a GP prefix, if it is, I would have thought it would be stamped as an Airsporter, it should also have a Walnut stock.
Mind you, it could be an older rifle, and I could be making a prat of myself (nothing new there). Some pictures could be useful. If you don't know how to host them, email them to me and I'll sort them out from here.
Cheers
Rich