I have approached the range question from another angle. Published performance figures state the IXD2 required 5,400 hp (presumably metric horsepower) to reach a speed of 19.25 knots. This translates to 1,308 hp for 12 knots. Assuming a total engine efficiency of 35%, the combustion cycle requires 3,737 hp, or 2,477 kW, to actually produce those 1,308 hp at the shaft. This comes from the fuel, which would have an energy content of about 42,000 kJ/kg. So, per second 0.0590 kg of fuel is used- or, 212.4 kg/hr or 5098 kg/24 hrs. With 442 tons of fuel, this translates to 86.7 days- or 86.7 days x 12 knots x 24 hours = 24,969.6 miles. So the range of 23,700 miles is quite possible. The discrepancy between the 24,969.6 and 23,700 miles is presumably caused by the fuel requirement for auxiliary purposes (or perhaps a slightly optimistic assumption for total engine efficiency).
For the Gato, Lenton's "Navies of the Second World War: American Submarines" gives a speed of 20.25 knots with 5,400 hp- presumably not metric hp, so the metric hp figure will be a bit higher. Assuming the figure is metric hp, however, gives a requirement of 1123 hp at 12 knots, suggesting the Gato's hull actually has less hydrodynamic resistance than the IXD2's, or a better propulsive efficiency, or both. Fuel capacity is listed as "389 tons except SS.228-235 & 275-280 378 and SS.236-239, 265-274 & 281-284 464 tons." Unfortunately no range figures are given. Assuming the range figures posted by Ducimus are correct, then the only explanation for the Gato's lower endurance per ton of fuel is the very large demand made by the Gato's auxiliary load (air conditioning, radar etc).
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