You'll need a current limiting resistance in series with the movement. Your direct connection probably explains why you blew up 2N3904's.
You mean between the transistor and the gauge itself? After the diode or the other lead? Resistance of the gauge leads is nil -- or less than 1ohm. That's the same as another of the exact same gauge I've not touched.
Does that mean I should be running between one of those gauge leads and the gauge lamp ground instead?
Hooked up to the boat's control panel this gauge had a yellow wire running across both of the leads and a white to both lamp leads.