VIN numbers are deciphered as follows. (Disclaimer: this was posted by a forum member, Hillary in '08

, in '03.)
Original post is here.
5 - Country of Origin (USA)
G - Manufacturer (GM)
R - Make (Hummer)
GN - Car line/Series (H2)
2 - Body Style
3 - Restraint System
U - Engine
Insert ninth digit here - check digit
3 - Model year (2003)
H - Assembly Plant (Mishawka, IN)
Insert 12th through 17th digits here - production serial number