I think this is the right order...
GTR - Standard GTR
GTR V-Spec - Suspension upgrades / updated ATTESA E-TS
GTR V-Spec II - Suspension upgrades / updated ATTESA E-TS / afew things revised
GTR V-Spec II N1 - As of above with upgraded turbos
GTR V-Spec II Nur - As of above, Nur short for Nürburgring / The N-Ring, famous track in Germany (I read that it was to comemorate the GTR being the fastest production car at that time... I think only in Japan... I gota read the mag I bought again, awhile back.)
GTR M-Spec Nur - As of above with upgraded turbos / leather seats (heated, I think) with gold trimings / softer suspension / champagne gold body colour (I read the "M" stood for Men, so Men Spec because it had a softer suspension system and was more a luxury car then racer... For Men.. ppfffftttt LOL)
GTR V-Spec II S1 (S-Tune) - Upgraded with Nismo parts / internal upgrades
GTR V-Spec II S2 (S-Tune) - Upgraded with Nismo parts / internal upgrades / tune for higher power
GTR V-Spec II R1 (R-Tune) - Upgraded with Nismo parts / internal upgrades / tune for higher power / Race Spec engine
GTR V-Spec II Nismo Z-Tune - Upgraded with Nismo parts / internal upgrades / tune for higher power / Race Spec engine / ULTIMATE Street car
GTR JGTC (Japanese Grand Touring Car)
I think thats how it goes with all the explanations... Someone correct me if I have missed something / anything...