The factory sender works as follows, completes a ground on one wire when the oil pressure is below 4psi. or shuts the light off above 4psi oil pressure.
Also has 2 other wires the fat ones so that when the car builds over 4psi. pressure the switch CLOSES the connection and the fuel pump will get power in case the fuel pump relay/ECM or that circuitry fails in the car it will still run after the engine cranks up over 4psi. pressure.
It's a redundant circuit.
You would best be using a T fitting with one end male threads the other two female threads, you can screw in the male threads into the hole in the block, factory switch into the end of it, and the aftermarket gauge into the side female fitting, aftermarket gauge adapter fitting (usually included with most gauges) will most likely be needed if it's a 1/8" NPT aftermarket gauge sending unit fitting.
You can use a T fitting with all female NPT threads but will need a close nipple on one end which is 2 sets of male threads on about a 1" long tube. That could screw into one end and give you a male thread output to screw into the block. Should you not be able to find the proper fitting as described above, one extra NPT connector used.
All NPT threads get a dab of teflon pipe dope for sealing.