The EPA hasn't yet rated the 2014 Mercedes-Benz G-Class for fuel economy. But, if you're concerned about gas mileage, you should probably shouldn't be looking for one of these in the first place.
In the past, the G550 model has been rated at a very thirsty 12 mpg city, 15 mpg highway. It hasn't changed mechanically, so there's reason to believe that the current model will maintain that rating.
That 12/15-mpg figure is the good news. The 544-horsepower, twin-turbo G63 AMG is likely to fare much more poorly on the gas-mileage tests, though standard stop/start technology could soften the blow on the G63 AMG's city-cycle economy.
The likely combined scores are the reason we've given the G Class a green score we typically reserve for supercars. It carries more than three times as many passengers as the average Lamborghini, but the G Class drinks gas just like an Italian exotic--or worse than one, depending on the model.