I checked all of the authorized Police codes thru 1937, there is no G in connection with P.O.. Putting a G there could possible indicate the Gumbinnen district of the Ostpreussen Police School, but I think that might be streatching it a little and assuming there was a school in the Gumbrinnen district. Here are the known listings...
http://www.radix.net/~bbrown/police_unit_marks.html