I've just fitted a multi function gauge but the oil pressure is not working, sender is in an adaptor with the oil pressure warning switch, oil light goes out ok but no pressure at all on gauge. How do I test the sensor or gauge is working? Sensor only has one wire and earthed through block, does it work on a variable resistance or voltage that I could test with a multi meter?

Might be worth putting a workshop type oil pressure gauge onto the engine first to make sure you have oil pressure rather than just enough to put the light out? Wouldn't be the first time an engine hasn't had oil pressure even though it's been rebuilt. Then check the resistance of the sender without the engine running and with the engine running and compare with the manufacturers spec.

Or take the sender out and attach it to a footpump/bike pump/tyre inflator and see if the gauge moves.