Although accurate it is not really the correct answer to the question posed.
The "N" body designation of previous Grand Am, Malibu (now Classic), Alero and....Skylark (LOL) actually shows up as 4th character in VIN of vehicle - carline
The corresponding value for the G6 (and 2004/5 Malibu) is "Z".
So this is the "Z" car platform.
Anyone interested in what characters of VIN mean? Here is a one specific for the G6 and Malibu
First a sample VIN (from a Malibu);
1G1ZT64884F217972 (hope this isn't anyone's - LOL)
Now what each digit means:
1= U.S.A. - country of origin:
G= GM - Manufacturer
1= Chevrolet (2=Pontiac) - Make of vehicle
Z= Malibu/G6 is "Z" carline
T= LS model (Malibu: S=base, U=LT and W=SS new for 2006; G6: G=Base V6, H=GT and for 2006 M=GTP and F=base with 4 cyl )
6= Maxx body (5=Malibu/G6 sedan, I forgot what coupe/convertible are but they are different)
4= Front/Side airbags (2=Front airbags only)
8= 3.5L V6 engine (F=2.2L L4; also forgot what 3.9L and 2.4L are)
8= check digit (calculated based on a formula)
4= 2004 model year (5=2005)
F= Fairfax, Kansas (Assembly Plant, G6's plant in Lake Orion, Mich=4)
217972 = sequence number