The 2.5g G.B.P. was used between 1898 and 1912 and it means the gun was proofed with 2.5g (38.5 grains) of flaked smokeless powder.
Under it, you should find St. M.G. which stands for steel jacketed bullet.
BARIBAL is absolutely on the money with this. I finally got some time to dig a proof marks book out of my Library to confirm it.
The proof marks on this rifle are GERMAN, before 1912. The 2.67 GBP is the charge weight of Gewehr Blattchen Pulver (Military Flake Powder).
I notice there is a dividing line under this GBP marking. If the rifle is disassembled there should be one of the three markings there.
St m G - Stahl Mantel Geschoss (Steel Jacketed Bullet)
K m G - Kupfer Mantel Geschoss (Copper Jacketed Bullet)
Bl G - Blei Geschoss ( Plain Lead Bullet )
If disassembled, there may be additional marks that are now hidden by the wood stock.
I also notice that someone has tried to disassemble the rifle without removing the locking wedge in the fore-end, and has cracked the stock in that area. A repair should be made after cleaning the crack, using thin epoxy or accuglass.
This is a fine Classic German rifle, and is definitely worth a bit of TLC and expense to make the old girl look her best, considering her age.
.