The JTD was by far and away the most popular engine in the GT and to my mind the best diesel engine you can get (against anything of comparable size).
Earlier versions of the engine were single cam, low on power and unsophisticated. Later versions become strangled by overly stupid euro regulations and daft devices to try and save polar bears.
The GT's 150bhp units were by far best of the breed with nothing silly on them to cause complications besides and the engine / ecu combination produced a really solid, smooth and very efficient motor, much more so than previous or later versions.
Thanks to its great power for its size, top shelf efficiency and very common application and parts availability the cars won a lot of friends in the company car market and enthusiasts alike, so there are a massive amount of GT JTDs out there to choose from.
Most people buy a car for economy so most people who wanted a GT got the JTD.
people that wanted power went for the 3.2 V6, but these suffered limited product runs, poorly timed UK road tax hikes and people often went for the 156 or 147 GTAs which could be got used and had a much more bespoke look to them compared to others in the range; so a lack of uniqueness or GTA badge for the GTA didn't attract people to the 3.2 GTs the same.
The 1.8 or 2.0 petrol options didn't really draw a crowd either as they didn't have any more power than the JTD (and lacked torque) and didn't have the efficiency either, so people didn't flock to them.
The only reason to go for a 2.0 petrol is selespeed if you fancy the challenge but too many horror stories from the 156/147 made these rare too.
Honestly - I like the petrols, 3.2 V6 is a great motor but if you want one a GTA feels more special but the torque from the derv makes them difficult to pick.
if you went for a 1.9JTD, stick an FMIC on, ditch a cat or two from the exhaust, LSD and good remap you will surprise some 3.2 boys anyway.