GBP stood for "Geweher Blättchen Pulver" which translates to "Rifle Flake Powder". This was the standard military powder of the day, but I have no idea what it might be close to in terms of burning rate in comparison with IMR powders. 2.75 grams = about 42.4 grains @ 15.43 grains per gram.
Miller


Miller/TN
I Didn't Say Everything I Said, Yogi Berra