Sending units can rust/corrode up over time and fail. That's more than likely the culprit. Check to make sure both wire connections are good first. Also I've see the gimble bolt loosen and pivot with the sender. Confirm both sides are tight.

Can also check wires with a volt meter with key on and just loosen the two screws and rotate.

Does it read the same with drive up?
If yes and
Assuming you don’t have smartcraft.
Find the 2 wires that come from the trim sender thru the gimbal housing and are tie strapped to the back of the engine. Disconnect them and put a short piece of wire between the 2 wires on the engine. Key on and look at gauge. If it now reads full down, you need a new sender. If it’s still up, post back and we can work thru gauge / wiring troubleshooting.

Pull it off and turn key on and turn the sender both clockwise and counter clockwise manually. See if you get any movement on the gauge.

The position adjustment is the puck on the starboard side. Odds are it’s time for new ones. The internals get corroded and fail. There are a few places where you can buy replacement internals if you want to try that. The actually sending unit can be bought new for roughly $70. It’s the R&R that makes it a chore because the drive needs to come off