The 12-year old GT from 2005 got the same overall mpg, and actually managed 1 mpg better ratings for the city and highway. The new GT's fuel economy number also means it will likely garner a Gas Guzzler Tax of $2,600. However, this tax could also be more or less, since the government uses unadjusted ratings that differ from the EPA numbers seen on Monroney stickers.
The new GT looks even thirstier when comparing it with its mid-engine rivals. The McLaren 570S, Ferrari 488 GTB, and Lamborghini Huracán, all top the American supercar in almost every fuel economy measure. All three manage to top 20 mpg on the highway, with the Lambo bringing up the rear with 21 mpg, and the McLaren coming out ahead with 23 mpg. The lead extends to city mileage, too, with the McLaren leading the charge again with 16 mpg, and the Lamborghini falling to the bottom with 14 mpg. And it's worth noting that two of these cars use turbocharged V8s, and one a naturally aspirated V10.
With that being said, no one that buys these cars truly cares about fuel economy. These cars are built to perform and look good doing it, not to sip gas.