I agree Scott completely, the theory of G for Gehr means nothing.
If 3B stands for 3rd Battalion, in the 1780 Berks County company returns, there was a 3rd Battalion (noted as “3rd Battalion-southern section”) whose Major was Jacob Kerlin. Published in “The Pennsylvana Associators”, 2nd vol. I know virtually nothing about the militia designations, other than what I see in an old Berks County in the Revolution book that shows the reference noted above. In 1780, August, John Kerlin Jr. was on the roster of 32 men in this battalion also. If the marking is legit, maybe the Kerlins were more prone to mark the guns they made for their own battalion? All speculation but interesting! If they even made muskets then!
The 3rd Battalion has rosters listed from 1775 through 1783. I believe 1780 was the only year with Major Jacob Kerlin noted. This battalion was associated with other geographic areas of Berks county in other years, such as Windsor Twp.