The first question is, do you have
oil pressure or not? DO NOT run the engine for a while to see what happens. Bad things can happen -
really bad things.
Remove the distributor and a valve cover and run the pump with a drill motor. You should be able to see
oil come up through the valve train to the top of the rockers.
If you have
oil at the rockers, and the gauge shows Zero pressure, the gauge has a problem.
That cheap plastic tube is easily damaged and pinched off, or start to leak. There should be some oil in the tube, but there should also be some air. Air is compressable, and will buffer down the oil pump pulses. Without the air, the gauge needle will be buzzing. Too much air, of course, and the gauge will be inaccurate and slow to respond.
If you spin the pump with a drill motor and don't get any oil to the rocker arms, you probably got the hoses switched at the block adapter. A very common error. Remember, center to center, outside to outside.