Looking at what you have said about the voltage ranges you need, you can see it requires a level shift AND a small change in gain, not an inversion.
The range of the Subaru control is -5V (throttle closed) to +0.5V (open) = 5.5V.
The range of the throttle sensor is +0.5V (closed) to +5V (open) = 4.5V
The Subaru range, if shifted positive by 5.5V, would put the throttle closed position at +0.5V suitable for the throttle sensor. That same shift would put the open position, previously +0.5V, up to 6V, which is somewhat over the top for the throttle sensor
Using a very simple op-amp circuit, called a non-inverting summing amplifier, you can apply bias, to give you the shift, up one input, and offer the available Subaru range up the other input. The gain can be set by the op-amp feedback resistor to make the output range become 4.5V. A diode with resistor can add a limiting feature to prevent the output going higher than might be safe.
This is in control of LPG fuel - so take some precautions. Use a small 5V regulator so supply the whole circuit, with it's input protected by a zener diode with resistor to limit the maximum that can ever come out of it, and choose the op-amp to be a type that operated right near the positive power input (called "rail-to-rail"). Use a polyfuse, a low-cost self-resetting component chosen to limit the current to the circuit. Add a varistor overvoltage surge protector (cost pennies!). Do not put up with failure in the supply. Use over-specified filter capacitors and some kind of choke to
keep transients out. Filter the output from ignition RF (3 small components + a resistor). It goes in a metal box, and give attention to the temperature. It does not have to live bolted to a hot engine part or suffering vibration.
All the above would fit on a couple of postage stamps. There are bound to be other ideas out there, and if this thread is left to itself for a few more hours, you might start getting suggestions. It is understood you may not have much electronics knowledge, but this is your chance to gain some, because a operational amplifier of this sort can be put together by a hobby enthusiast, or school class exercise. If you get no other help, I will try and make a quick sketch.
P.S. It occurs to me that we need to get clear which end is at the iNPUT.
Is it the throttle sensor that has to deliver to the Subaru? .. or is it Subaru voltage that has to be changed to deliver to the sensor?
This immediately raises the question, where does the negative voltage come from? Surely the Subaru does not have its positive battery pole connected to the chassis! ??