That is strange. According to the GM Vehicle Info Kits, thru the AD years, 47-55.1 the only plants assembling trucks were:
1. - Flint, MI
2. - Tarrytown, NY
3. - St. Louis, MO
5. - Kansas City, MO
6. - Oakland, CA
8. - Atlanta, GA
9. - Norwood, OH
14. - Baltimore, MD
20. - Los Angeles, CA
21. - Janesville, WI
I even looked at 46 and 41 and still no plant Number 7.
GP is definitely the model code for a 1949 3100. I assume you're talking about the serial number plate on the door pillar. Is it possible it could be a partially stamped 9?
Do you know where the truck came from? Since you're in Maryland, it would make sense for it to be a Baltimore truck, unless it came from another part of the country.
Maybe someone else can shed some light on this mystery.
