I have always heard that the gas light comes one when you are around 2.7 gallons remaining... I know in my old Dodge Dakota, that is what was mentioned in the Truck Manual. I haven't looked in my current manual to see if that is mentioned, but its what I have based my understanding off of the past 10 years.
2013 Dodge Ram 2500 CC Diesel
2014 Jayco Jayflight 32RLDS