The stamp >>2,5gGBP<< equates to 2.5 grammes (Military)rifle flake powder points to the Standard Service Charge for the Semi smokeless 8x57 cartridge. >>StmG<< is the bullet type. As this mark was only used up to 1912, it probably indicates an I = .318" bullet bore. You will probably find the following proofmarks under the barrel: a big crown, a small crown over N indicating nitro proof using the 4000 ATM special proof powder, and the gauge number 172,28. Only if the gauge number is 156,14 it may be a S =.323" barrel.
If any of the of the bore gauges are present, it would date pre-1912.