I have a gas station and have been using RMS for a week. Not an expert.
I believe GAS PUMP is just a feature to help the cashier remember how much money the customer paid upfront to fill up a car when he does not know how much will it take to fill it up. It speeds up the TENDER when you recall GAS PUMP on the lower section of the TENDER window. It inserts the PrePAy amout on the CASH block of the tender window so the cashier does not have to type it again.
You must set the item GAS to type "G" (Gasoline) so that when the cashier ring the GAS item, it will allow you to enter the total dollar amount on the extended price. The POS will then recalculate the quantity based on the price per unit set for the item.
My pumps can serve PREMIUM or REGULAR and the customer makes the choice. My cashier can only activate the pump but can not select the type. Therefore, the feature of recalculating the quantity based on Extended Price is of no use to me because I have two different prices and cannot anticipate the customer selection. However, works OK for the Z Report because I can check the Gilbarco report against the Z Report based on Total Dollars and not on liters.