Also keep in mind, the gauge may very well be working just fine. The only time the oil pressure would drop from low oil would be if it was low enough to starve the engine, and it usually needs to be much more than 1 quart low to do it. The oil pressure should be the same whether it'* a quart low, or full.
As was said however, you can pull the sender, stick a mechanical gauge on it to verify readings, and if the readings are wrong, then replace the sender. That'* even assuming that the gauge is even designed to give a reading. There are plenty of gauges out there that have about the same functionality of an idiot light.
What weight of oil are you running?