What account do I setup for gasoline? There is no monthly balance. I use my ATM
I'm not sure you mean account, but probably category. It doesn't matter what account you pay with, but when you enter the transaction, you would put the cost toward your fuel category.
And of course you would take money from your TBB (To Be Budgeted) or transfer from some other category to initially fund that category.
The account is the payment type: where the money you pay with comes from. So you might have a Cash account, a checking account, a savings account, a credit card account, etc (as many as you have real bank accounts). If you withdraw cash from checking you would enter a transfer from your checking account to your cash a count whenever you withdraw money from the ATM. There is an "add accounts" button on the left hand column of the web app, or at the bottom of any existing accounts for mobile.
Gasoline would be a category on the Budget screen. When you buy gas, the transaction would be in your Cash account with a category of Gasoline and a Payee of your gas station. If one day you didn't have cash & used your debit card you would enter the transaction in your checking account instead.