Greetings to both of you, and welcome to the Forum.
Lazerguy may be right, how much Fuel does it take to fill it up, from that point.
You know how much Fuel the Tank will hold, and if you have to put a full load in it, then the Gauge is wrong.
But,
If you only have to give it half as much as the Tank is rated at, there may be a break in the part of the Pump that goes down into the bottom half of the tank. Somehow, it's drawing Air into the system, or, allowing the Pump to loose it's prime.
The quantity of Fuel that you have to replace to get it to run, will tell you if you need to replace the Pump. Half a tank, then yes. A whole Tank, then it's the Gauge.
Have a good Day !
S.W.