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North UK

Okay i see the dash, it has the mechanical fuel sender

Not sure if it was done before, but if the cable is removed and the front wheel drive has been cleaned off foreign debris,  refit it all back & go for a quick test ride.

if not, my bet would most likely be that the speedo drive could be replaced for another ,  if it was new as you said,  then the next process of elimination would be the cable itself ,  before attacking the dash last.

Fuel Gauge,
You can remove the whole instrument cluster off the steering base, be sure to watch the cables.
Loosen the instrument cluster from its own mounting bracket too, as it helps gain access to the fuel gauge under it.
The gauge has 3 wires and short plug going to it,  it should come out on its own under the dash for inspection.
With the fuel gauge unit exposed on the table,  gently (& slow) push the needle up then see if it comes down on its own fast or slow without hesitation ?

Also clean the terminal plugs if there was signs of debris. Refit the whole dash back.
As you ride, see if it still behaves that way again?

You can then drain the fuel tank, look at the fuel sender inside the tank, take it out.  Some have a small bolt attached to one of the linkages of the float, tighten a little and refit it back.
Clean the plug of the sender to incase of any rust or debris in terminals.  The sender’s resistance signal changes as the fuel goes up then towards empty, this tells the dash what level to tell you its at, so good condition wiring helps near the terminals.

There is about 3 types of senders in all GT (any cc) (IIRC), some have a long arm float , some have a big spindle type design – You would only know when its taken out.   Just be sure to keep the main seal clean from any future leaks, it can be hard to find on its own lol.

Your last resort likely be a replacement of the gauge (dash side) or level sensor (tank).  Age can wear down some moving parts, always good to have a look.

// Meditation doesn't mean you have to sit still....