the only TDI's i know of that are badged as GTI's are either the 150bhp model of the Anniversary model.
for the rest it mainly depended on what badges the dealer had in stock!! but i reckon as a general rule of thumb the following applies
TDI - all silver is the 90 or 110 models (might be others)
TDI - red I indicates the 115bhp model
TDI - with red D & I is the 130bhp
TDI - all red is the 150bhp
GTI - is most likely the Anniversary model which also had the 150 engine
then you see some badged as GT TDI..........
more than welcome to badge it as a GTI as its quicker than the 2.0 and 1.8 n/a GTI models
