First Guess: The oil pump pickup tube has fallen off the oil pump. Your "mechanic" tested the unit at rest, not with the oil sloshed forward, away from the oil pump.
You could verify this at your next oil change, by pulling the oil pan drain plug, allowing all the oil to flow out, then poke a borescope into the drain plug hole. Take a look around inside the pan--if the pickup tube is lying on the bottom, you've found the problem. This assumes that the pan has the proper amount of oil in it now--
a very low oil level can cause this problem, too.
Another possibility is that the wiring or the connector on the end of the wire for the dash gauge is faulty. Heavy braking causes the wire to move forward, and the broken conductor opens the circuit. Some time spent inspecting the wire and repairing as needed may be all you need.
If you insist on installing a "mechanical" gauge, you'll need to figure out where you want to put the gauge; you'll need some sort of mounting bracket. The gauge comes with instructions for how you connect power from the dash light fuse, and ground so that the gauge lights up with the rest of the dash gauges and the brightness is controlled just like the rest of the dash--via the rheostat near the headlight switch.
At that point, it's just a matter of getting a brass "tee" to screw into the engine near the distributor. You'd remove the existing sensor for oil pressure, install the tee, put the original sensor into one branch of the tee, and the plumbing for the new gauge in the other branch of the tee. Route the tubing through some hole in the firewall, PROTECTED BY A RUBBER GROMMET so the sheetmetal can't abrade the tubing and cause an oil leak. The tubing--usually nylon, but some folks upgrade to copper or steel-braid-over-Teflon (PTFE) liner hose--MUST be secured so that the engine can shake without stressing the tube/hose, and yet the tube/hose doesn't rub or abrade whatever it's routed near.
I put a tee into my 5.7L engine. I don't have a photo of the sensor and hose connected, though.
The brass tee is behind the "Sch" of my name on the copyright notice.
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