Gas mileage isn't usually a big GT's ally. The 2013 Maserati GranTurismo doesn't change that pattern--it's not much different from the Jaguar XK or Mercedes-Benz SL in its low fuel economy.
The EPA rates the GranTurismo coupe at 13/21 mpg, and 16 mpg combined. The GranTurismo Convertible weighs more, and its numbers fall to 13/20 mpg or 16 mpg combined as a result.
On a firsthand note, we've observed figures as high as 16 mpg, even when charging hard through a desert canyon or two in the GranTurismo coupe, but that's probably the most you'll see with regular use--especially if you keep its V-8 in the best-sounding part of its powerband.