I think the G1000 installations use a fuel totaliser which in GA is usually a Floscan 201 turbine transducer, feeding what is basically a pulse counting instrument. A bit like
this stuff.
What I found curious is that not one of the pilots I have ever spoken to who flies behind a G1000 knew anything about the system. So maybe the totaliser function is optional? But if it is missing there is no way the system should be showing what appears to be a "fuel on board" figure because it will be way off no matter how derived.
There are also installations which never read right. These normally result from the transducer being in the wrong place, or the wiring being unshielded and picking up e.g. ignition interference.
Example
Assuming a proper totaliser is installed, you indeed have to calibrate the instrument for the actual "K factor" (as Shadin call it) which you determine according to the pump fillup, following starting with the tanks topped-up accurately to the brim, some flying, and another topup to the brim. You also need to do it at a pump which is regularly checked by Trading Standards, which rules out a lot of airport bowsers and rules out many fixed pumps in N Africa (i.e. anywhere south of the Alps or the Pyrenees) as those pumps tend to be adjusted for a few % gain to the airport.
It cannot be done legally by the pilot, which is a massive irritation because one usually has to do it several times to get it just right. I used to fly to Air Touring to get it done
In reality most people with the old instruments just do it themselves. On the Shadin instruments it is either rotary hex switches or a front panel adjustment (and you need the easily obtainable installer manuals). I have no idea what it is on a G1000; presumably it is in one of the installer pages. I have very recently acquired a ton of G1000 install manual PDFs from some kind soul but haven't bothered looking through them....