Have them to check the tank breather (or whenever you call it in english) tube, it can affect the refueling (the pump will stop believing the tank is full) and eventually the fuel sensor.
I suspect you might have an issue with the transfer pump. Our cars have 'saddle' tanks, so the fuel from the right side (I believe) has to be transferred to the left, and I've seen many of those transfer pump failures lately, leaving owners stranded with no fuel, with similar gauge issues as yours. Make sure to reset one of your trip-meters after each refuel, so you don't rely solely on the gas gauge. I've been running my tank below half since I bought the car, and it's still working fine, so I assume my transfer pump must be good. Several of those folks who reported a failure always refueled with about half tank, so maybe that had something to do with that (never worked). Who knows. At any rate, good luck, and let us know what it was.
Got back from dealership yesterday, they're ordering a new sender unit from the states and will fit that in, apparently two other gt's have had similar issues at this garage.
I think our cars have TWO of those, right? But it's a better problem than the transfer pump, since at least you'd be able to use all the fuel in the tank. Hopefully fixable from the top of the rear seat, so tank doesn't have to come down, which I think requires removal of sub-frame, suspension, and differential, but not sure about that. Good luck, and keep us posted.