There seems to be quite a bit of bluing on those they've got hot and then cold very quickly for that kind of metal fatigue to manifest itself. Purely from a curiosity point of view did the crack appear after the disc had cooled or in operation
Your GT calipers, are they the ones with the internal connecting pipe or do you have a pipe under your caliper.
If you have the newer design then it is possible that you have corrosion behind the pad slide plate, its caused by differential cooling between the caliper and the steel screwed in slide plate which the edge of the pad slides down. The problem is a build up of dust and probably pad material behind the slide plate, this aint a problem in itself until it starts to corrode the ali caliper and the steel "bulges" preventing the pad from withdrawing. Its amazing how much force corrosion can generate.
Only thing is a full rebuild really, alternatively it could be a sticky piston - in either case it wont do any harm to strip down the caliper remove the corrosion, etch and re powder coat the calipers. If you do have a sticky piston then they are easily replaced from Porsche dealers about £25