The resistor that drops the voltage to the heater will act like a dead short on the way to the gas gauge. You'll need a totally different resistor to handle the tiny current draw of the gauge, with a lot more resistance that the one used for the heater motor. If you don't have a pretty extensive knowledge of electronics, you're going to be spinning your wheels trying to make the gas gauge work. I'd suggest a voltage divider circuit using a potentiometer that applies just enough voltage to make the gauge read full with the float at the top of its travel, and empty as it reaches the bottom.
Jerry

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