What happens when you change the trim setting? Any needle movement at all? Chances are the gauge itself is not bad but the trim sender on the engine is.
You can connect the gauge to a potentiometer then moves the dial (very slowly) to see if the needle on the gauge respond to it. BUT....... like silvertip said, it is very very likely the sender is bad. You can check the sender with a voltmeter. If you don't get any changes in the resistance value when you move it, then it's defective.
i never have seen one of those damn things work....it's always in one direction and stays there and that is on 2 brand new boats...my dad's never has moved much on both his boats and mine doesn't seem to do anything either
