If you are in Brisbane, save your travel dollars for a closer Gem and take it out for a sail.
Less buoyancy because of fresh water causing a speed reduction? BS.
Ask for a measurement from water to top of deck at the center most stanchion, starboard side and PM it to me. I'll take mine the same place today.
I'm cruise loaded as heavy as I get, half load on water and fuel right now. I'm showing 3 inches of original black bottom paint above the water amidships, less aft because of my dinghy, and full lockers aft (extra batteries on port side yada yada). My stbd cabin is remodeled as a pantry with 200 lb plus of pots, pans, soda's, and food. Port cabin is general storage, 200 lb plus of chairs, tables, grill, and various stuff. I carry a lot of extra back aft; I'd balance it out but prefer the bow to be light so I don't bury it in a power boat wake.
I'll repeat, flat water, no wind, 7.2 knots at full RPM - did that just last weekend. 2800 rpm; 6.0 to 6.2 knots depending on wave and wake.
I'm not trying to kill the sale of this Gem, it's probably still a sound boat. My thought is the hulls got out of alignment when it capsized and that is slowing it down, or - having given it a lot of thought:
Gemini's aren't exactly built to high standards. There is a lot on non-encapsulated, non-sealed wood used in the construction. A capsize would have soaked it - it might be wet and heavy. It's been years, it might have twisted when drying. Other than weighing the boat when empty I don't know how to check that - and weight can vary. PCI says either 8,600 or 9,600 depending on who you ask.
It's a lot of might. I'm not a surveyor.
If you want to reach me, best way in on Anything Sailing Forum