Welcome aboard.
If it's VDO chances are it's the European standard resistance. Hopefully it's a defective sender because its likely the gauges aren't available anymore unless you can find them on-line through someone like
https://www.greatlakesskipper.com/Don't forget you have access under the helm through the back of the forward bench if I remember correctly. From there, you can do a couple of other checks in addition to the one G36 mentioned. If that test doesn't tell you anything, don't be too quick to condemn the gauge or sender before checking your wiring, harness plugs, etc. If I recall, it's one gauge with a switch to check two tanks? If so, you want to confirm there's continuity through the switch too.