A G159 First Officer reported noticing a Master Caution warning 'R GEARBOX PRESS' during cruise flight on an overwater leg. Although the checklist called for feathering the engine; the Captain decided to keep the engine running.

Narrative

On the day in question; I served as the Second-in-Command aboard a Gulfstream 159. We were established in flight on an overwater charter flight established in the block FL210-250; holding FL220 approximately 2 hours into our route. While in the course of my duties; I observed the Master Warning light come on on the eyebrow dash. My right side Master Caution Panel indicated 'R GEARBOX PRESS'; meaning right gearbox pressure. The light was intermittent and remained intermittent throughout the remainder of the flight. Within a second or two; I notified the PIC of my observation and we began pulling the emergency checklists. I called out the appropriate emergency checklist; which said in one line 'Gearbox Low Oil Pressure: DURING CRUISE: Accomplish Manual Feather Checklist.' The PIC placed a satellite phone call to our home base. The PIC was unable to reach anyone for several minutes. We were located approximately 480 nautical miles from destination at this point; and I was afraid to override the PIC's determination not to immediately shut down the engine in flight. The PIC did eventually reach someone at our home base. I was not privy to that conversation because the Satphone only plays into one headset - so I can only hear the PIC's conversation. I later learned that the PIC was instructed to shut down the engine. After finishing the phone conversation; the PIC informed me that he was 'not going to shut down a perfectly good engine in flight' and I presumed he was aware of facts that I was not privy to from his discussion with our maintenance and management. I radioed Center and requested and was granted clearance back. I answered all questions to the controller truthfully. We were able to reach departure airport without further incident. I am going to engage my company regarding emergency procedure training and I am going to thoroughly review all systems on the aircraft to assure that we are fully complying with the manufacturer's recommended practices.