The alarm uses sensor switches that are independant from the helm gauge sendors. Try disconnection one of the switch sendors at a time until you find which one is at fault. From what you have described it is most likely either a gear lube or oil pressure switch. Is the gear lube at the full level in both reservoirs? Even if it is sometime the floats that activate the switch can stick in them.
Find the tan/gray wire and disconnect it at each of these them operate the boat for a period of time until either the alarms sounds which will tell you it's not the disconnected one at fault or if the alarm no longer sounds you found the culprit.
1997 2400 SCR
5.7 Vortec / Bravo 2