Walmart gift cards are meant to be used to purchase things in the store of Walmart or online at Walmart.com
You cannot use a Walmart gift card to pay any bills other than buying things from Walmart.
You can use a Walmart gift card to buy anything in the Walmart Store or online at Walmart.com including gasoline or diesel fuel and cigarettes as well.
You can usually sell your Walmart gift card or other gift cards online for cash or money deposited into your Paypal account which you can then use the money to pay a bill with.
But if not then you cannot use the Walmart gift card directly to pay a bill with other than paying something at Walmart.