Think your getting confused
GT is worth it if like me you can't be arsed with aftermarket ICE, like having leather heated seats and can find a use for cruise control. Personally I wouldn't pay the £2k extra if I was buying new but if I was buying a used one I'd want the highest spec available for my money.
For me to buy a non GT I would have to pay about £500 for a leather retrim and probably about the same for a decent ICE kit and for it to be installed.
If your going to keep it standard, go GT. If your gonna mod it yourself then get a non GT but make sure its cheap as you'll have to sell cheap too.
The GT4 was a limited edition of which all come with the GT pack. the 100 black ones are only different to other MY03's by the extra 20 bhp, rims and slightly different side badge which are barely noticeable. The other 76 has these features plus they were yellow. Therefore, in my opinion, if you want a black one then get a MY06 black one, if you want a yellow one then it has to be a GT4
EDIT: must be my longest post in months