I have a 2.0l GT, and had two 156 2.0l before it.
The GT is AFAIK based on the old 156 technology rather than the newer stuff.
I think it is based on a 147 floorpan, not the 156. It is stiffer than the 156, although that might not be such a good thing as my 2nd 2.0l sportpack 156 was like a limpet on the road and my 2.0l GT isn't as good.
Find a specialist for servicing, not a main dealer.
Consider wheel size. Mine has 17inch alloys and the wife doesn't like the comfort, on 18 she really didn't like it and I have ssen complaints about comfort and tramlining from 18inch alloy wheels.
My sp 156 had 16inch alloys, handled better and was more comfortable.
my 1st 156 had 16inch alloys which were wider than standard (it was non-sportspack) and handled well (not as good as the 2nd one) but was very comfortable for the passenger.
I swapped out the Pirellis that were on mine for Goodyear Eagle F1 asymmetrics which are much nicer - seem to consistently dislike Pirellis on the alfas and didn't much like the bridgestone potenzas so3s either. Dunlop Sports were better.
I prefer the petrol engines - the 2.0l in the GT is better than the twinspark in the 156, although maybe doesn't sound as good. More torque at low revs, which the 156 was poor at. Get about 390 out of a tank of V+.
My Alfas have used oil, so you need to keep an eye on it. Apparently not a fault, as Ferrari engines are built to use oil, I hear.