No oil to the top is often caused by the head gasket being inverted, blocking the oilway. As the noise was there before the gasket was changed, it may have been done before, and your technician replaced the new gasket in the same way as the old one.
Or:
Oil pipe is blocked.
Oil feed up through block to head is blocked.
Pressure relief valve in oil pump is stuck, reducing oil pressure.
If the oil is not changed regularly, it gets thick and sticky, blocking the oilways.
If the oil pipe over the tappets is removed, does oil then feed up to the head? If so, needs a new pipe, or clean it well.
If no oil supply, head off again to see if oil is fed as far as top of block, and ensure gasket does not block the oil feed.
If no oil up through block, it gets more difficult. Oil feed through block is more difficult to clean out, unless engine is out and stripped.
You might be able to ask previous owner about recent history, but if he knows about the problem, he is unlikely to be honest.