I'm reading that Italy usually closes gas stations on Sundays. If so, were do you have to go to buy gas? I'm guessing that the major freeways would still have (some) open stations, but don't want to bet on it.
The freeway stations will be open.
You can buy gas with cash at many of the unattended stations, and some of them may take credit cards. Our credit card did not work, but they had clear instructions, in Engish, on how to put 5 or 10 or 20 EU's bills in and get fuel.
We had to do that to get by on Sunday when we were in a town.
You can also watch an Italian do this, and ask for their help if you cannot figure it out.