the domain contains all values of the traveltime t. Because time never can be negative (there are a few exceptions which will be explained to you by the SciFi-section), therefore t must be greater or equal than zero. The traveltime ends abruptly when the gas tank is empty. Thus:
The range contains all values of g. The tank is full if 16 gallons had been filled in and it is empty if there are 0 gallons in the tank. Thus:
I've attached a diagram.