If the oil quantitry is "inadequate" the oil pressure gauge will give no warning until it gets sufficiently "inadequate" to cavitate the oil pump. At that point the oil system pumps air to the main and big-end bearings and the engine is (to use the technical term) "completely phucked". If there is a blocked oil gallery the oil pressure gauge won't show it because the back pressure will simply cause the relief valve toi open further and dump the oil into the scavange system. In these circumstances the engine will also be toast, but it may last a little longer than it would with a cavitating pump.
The main failure modes that the oil pressure gauge will indicate are an oil pump or pump drive failure and worn bearings. Oil pressure that starts in the normal range but drops to low levels when the oil is warmed (especially if iit's very low at low or idle revs) usually indicates imminant failure of big-end bearings or slightly less (but equally terminal) failure of the main bearings. Either arising is a "land in the nearest suitable field and be prepared for a sudden complete enegine failure" scenario.
PDR