That's kind of a generic question, as it depends on many factors. First off, tire brands DO vary in width, even for a given dimensional size. Wheel width will alter the tire's width too. Tire diameter will greatly affect how wide a tire can be run without rubbing. Wheel offset also affects maximum usable tire width too. And lastly, the ride height of the car will greatly affect the tire clearance. So in other words, it depends on a lot more than one factor, but rather the sum of all these factors.
I've probably been utterly useless here, but you can see that it's hard to comment without all the facts in front of me. With more info, maybe members of this forum can comment on the largest tires they've fit to their GT's?