The body is Ford, everything else is Mazda. My daily driver is a 95 GT. I get my parts from Mazda. It's got 145k on it and I've had no major problems.
The motor is decent and not too bad to work on. It's probably going to be out-classed in ITS, though. Sport Compact Car did a build on one a long time ago and had a hard time getting it to make good power in NA form. A lot of people stick a turbo on it, which seems to work well, but that ain't gonna help you in IT.
The suspension on the GT is better than the MX6. The MX6 came with 215 tires and a softer setup. The Probe GT came with 225s and a stiffer setup. You could probably get it to handle well, but not like some of the other ITS cars.
You might be able to make it a front 1/3 of the grid car with a top build, but I don't think it could compete for wins with the other stuff in ITS.
David