I was wondering the difference besides price and I found this review: and the GT's are $100 more for the set
While the tread pattern looks similar to the Angel ST it has been designed to the channels flow from the centre to the outer edge of the tyre, designed to move water away faster. The profile has also been changed so it's now wider, with a 14% larger footprint when upright than on the Angel ST. Pirelli also claim the new profile allows the Angel GT to offer 5% more lean angle do to the profile of the tyre not dropping off a cliff on the edges.
The tyre's carcas is also stiffer which allows the rubber compounds on the edges to be softer. The front tyre is all one compound while the rear is two. The central compound is 70% silica; a harder rubber for higher mileage while the shoulders are 100% silica for better grip, especially in the wet.
Pirelli claim the Angel GT offers 30% more mileage than the Angel ST. Based on some sums on the back of a beermat and depending on your riding, you're looking at anything between 5,000 and 8,000 miles.
Essentially it's an Angel ST with better wear and wet weather characteristics.