I assume it's the Gray Plastic Y-Valve located behind the
head in the bow of the port
hull. I don't
recall any other Y-Valve on a Gemini--or at least on the one we chartered a couple of years ago to check the
boat out.
Unfortunately, it's likely the former owner rarely if ever opened or closed the valve. Some owner's prefer to simply
pump waste off-board regardless of what the law is or where they're anchored while others always use the
holding tank, even when it's not necessary (as was the case on the
boat we chartered). Absent working the valve periodically, they do freeze up.
N'any case, you may find that by
heating up the valve with a hair dryer set on high and held 6"-8" or so from the valve, the heat will expand the outer shell enough to allow you to get the valve to move, even a little. (Don't try this with a heat gun as they put out too much heat!) If the valve loosens even a little, you can "work" it back and forth and slowly restore the action. Once it's moved a bit--try running a botttle or so of head lub mixed with warm
water through it and you may be able to free the action up. Regrettably, replacing the valve may be necessary if it's thoroughly scaled up with
salt and calcium carbonate.
Good luck!
s/v HyLyte