Digital Control of Analog Gauge
For some reason the sender links to pressure senders on the datasheet, so I'm looking at a different product. Here's their calibration table:



The gauge has 3 terminals, ground, +12 and signal and the sender has only two so the sender acts as a resistor between the "signal" wire and ground. 

We can be pretty sure that the unspecified voltage on the signal wire is above ground, so a PWM MOSFET to ground, with a resistor in series, will likely simulate it well enough. 

So supposing the 250°F sender calibration point is 30 ohms, take a MOSFET and add 30 ohms in series. Power rating unknown, but worst case it would be 6.5W (on 100% of the time). You could try a 10W resistor to start with. Quite possibly much less than that (my guess is a 2-3W is enough). Wire it up with a 30 ohm resistor to ground and measure the voltage, that will tell you the power dissipation requirement for the resistor. With a 13.8V supply it should read 250°F. 

Build a PWM circuit (maybe a few hundred Hz) and adjust the PWM until it reads exactly on each of the marks, noting the percentage. I would plot it to see how nonlinear it is. Fit a curve to the different percentages and you should be able to make it read intermediate points (relatively) accurately. 

This is assuming an electromechanical gauge as shown, if it's electronic it might require some tweaking.