Believe it or not but here is the 2-door or 3-door coupé explained. Source: both cars’ sales technical spec. brochures, bodywork-dimensions sections:

Alfa GT is a 2-door 5-seat coupé
(Alfa Romeo GT Technical Specification brochure 02/07. Quote: No of seats: 5, No of doors: 2)

1024px-Alfa_GT_rear 1024px-Alfa_GT_Facelift_rear gtdimensions

Alfa Brera is a 3-door, 2+2 coupé
(Alfa Romeo Brera Technical Specification brochure 04/08. Quote: No of seats: 2+2, No of doors: 3)

Alfa_Romeo_Brera_(6883900965) 1024px-Alfa_Romeo_Brera_Rear_20070321 brera-dimensions

Question is why does the GT classiffies as a 2-door than Brera? Because it’s less like a hatch and more like a half-box notch. Brera is more a 2-box coupé and it’s quite clear.