I'm looking at buying an oil pressure sender online from one of the OEM parts sites (usatoyotaparts, toyotapartscheap, etc.) and they list units with, and without gauge.

Oil Pressure Sending Unit w/ gauge ~$45
Oil Pressure Sending Unit w/o gauge ~$30

Is "gauge" the actual electrical pick-up on the instrument cluster, or something else?

Possibly some Sequioa's have an IDIOT light instead of an actual gauge? My initial feeling is that the sending unit w/o gauge is for a vehicle that has only an idiot light and the sending unit w/gauge is for one with an actual gauge that displays oil pressure. I'd recommend you go to Toyota Parts - Genuine OEM Toyota Auto Parts and Accessories (http://www.toyotapartszone.com), enter the actual VIN for the vehicle, and it will confirm if the part fits your vehicle, so no guessing necessary.

@marvbest Feedback from Customer Service where I ordered agreed with you. "w/" is for if you have a gauge and "wo/" is for a warning light instead of gauge.