For general Alfa buying advice, Grahameo's post on this thread really can't be beaten: http://www.alfaowner.com/Forum/gener...purchaser.html (Any advice for a potential Alfa purchaser?)
I've had my JTS engined 156 for nearly 3 years now and actually can't fault the JTS engine. However, I bought it with a full service history that had been carried out by an Independent that I've used for over 20 years.
The JTS is a direct injection engine so it's more powerful than the twin spark (165 v 150bhp) and more torque (206 Nm @ 3250 rpm v 181 Nm @ 3800 rpm). The JTS pulls strongly right through the rev range put perhaps doesn't have as much character as the twin spark. Early JTS's (pre April 2003 manufacture) were noticably down on power, this was resolved with new pistons, ECU and exhaust cam.
Fuel consumption is pretty good, I'm getting about 35 mpg around town and rural A/B roads, on a long run I'll get around 45.
The down side is that the higher compression ratio means that oil consumption is higher than the twin spark. I'm getting around 1600 miles/L using 10w60 oil.
Clogged up injectors can be a problem on direct injection engines, mine's been OK but comments on here would certainly indicate that it's an issue. On a test drive I'd expect smooth power delivery through the rev range with no flat spots or hesitancy.
JTS engines have 4 lambda sensors and the wiring to them is a bit iffy. If you have the means of cheking fault codes it would be worth a look before buying.
A faulty thermostat is also quite common, so check the engine is getting up to temp on a test drive.
That's pretty much it on the JTS side, it's certainly worth looking out for a well looked after example. I can't comment on GT specifics but I'm sure someone on here can help you out.