Not the complete answer I know but.....
L = lights(To the light switch).
G = Ground (Earth)
S = Sender (To it)
I = Ignition switch(In the 'run' position)
O = I'm not sure about
If you have a multimeter, without connecting the 'S' to anything, with the ignition on
you should put one terminal from the multimeter on 'S' and the other to earth and
you should get a reading with it set at '12v'. If you do then the sender may be faulty
or incompatible with the gauge.
One more thing you could try is to email Mooneyes or Classic instruments with a full details of your gauge for your connector details and they would should be able to give you details of a suitable sender.