The resale value will be affected because some driver’s will not buy a car with a defective fuel guage. It’s hard to say how much impact it will have. You should get an estimate of the repair.
I sold a Volvo with a defective guage. The car had 2 fuel tanks and the repair estimate was $1,200. I just reset the trip odometer every time I bought gas. The buyer of the car was OK with that, but 2 others who looked at the car won’t accept it that way.