Without lifting up the Bonnet (Hood):
No Brembo brakes and Multifunction Display Unit (MFD)
GTT's have 3 gauges in the middle of the dash, GTR's have an electronic MFD, a screen.
The stock body kit and spoiler is all different.
Wheels, Badges, Bonnet all different.
Front and Rear Guards are a tad smaller (Less Wider)
GTT = Auto / Tiptronic / 5 Speed
R34 GTR = 6 Speed
GTT = Side Mount Intercooler
GTR = Front Mount Intercooler