Sadly i don't know the figures, it will also depend on how hard you are running it, be it load and the speed at which you are running it. Figures i have seen on the net show 1.3-1.4 gallons/hour BUT that was also under no load, so not real world figures.
Have you looked into what's wrong with the gauge? It could be the sender or the gauge it's self. If you take the panel off and short the S connector to ground, the gauge should read full. If it doesn't move, there is a good chance the gauge is bad.
The sender is the last resort, it's not in a great location to remove.