313. notes a 6,25mm pre-rifled plug gauge & since it has the voluntary Nitro stamp(July 23rd, 1893) with a designated powder type over a bullet type, I'd say it was made between 1895 & say 1905. I would guess it was made in Zella-Mehlis even though it lacks a date a ledger number, which may have been used a little later. GBP is for Gewehr Blättchenpulver). Any stamps on the frame or standing breech? Look closely near the forend lug for initials.

Kind Regards,

Raimey
rse