In one case you have a linear relationship and an inverse non-linear one. It seems like someone should make a universal solution at least the ability to adopt senders to existing gauges.
In one system I knew of, there was a pulsing regulated voltage and a fluid filled ammeter. It's also why, the gauge doesn't bounce around and it takes a while to register when first turned on,
Liability is the main reason, I'm sure people don't take to this project well.
There's another way that's used an it also compensates for low battery voltage.
Measurement of open circuit voltage and short circuit current and any waveform present helps to get started as well as if there are any "regulator modules" and/or multiple terminals on the gauge.
I have been contemplating a similar issue, but not with fuel gauges.
A microprocessor is likely the "way to go". Your problem is way simpler than mine.
I think when you mean "LPG level gauge" and "fuel level gauge" you mean sending unit or simulated sender.
So, in effect, you have to "measure the sender's" resistance and convert to a current source (voltage across a resistor) through a look-up table to drive the existing gauge.
Unfortunately, you can't assume much.
An Arduino would surely have the smarts to do so.