If you are deducting fuel costs then you would simply enter your gas costs as an expense for the vehicle. It is not part of the depreciation calculation. As you walk through the screens to enter your vehicle information, you will come to a screen that asks if you want to take the actual cost or standard mileage deduction. If you choose the Actual costs, you will then be given the chance to enter all of your fuel costs along with other expenses such as tires, oil changes, etc.
Unless you are operating some type of fuel business or carrying fuel as inventory, you would not enter gas as a supply