Yes a Walmart gift card can be used at Sam's Club gas stations.
Walmart and Sam's Club are owned by the same companies so Sam's Club does accept Walmart Gift Cards for gasoline purchases as well as other purchases inside the Sam's Club Store.
I've used Walmart Gift Cards before a lot to buy gasoline when at Sam's Club Gas Stations and also used the Walmart Gift Card to make purchases inside a Sam's Club Store.
You can use Walmart Gift Cards at any Walmart Owned Store or Walmart Owned Gas Station and even at a Murphy's Gas Station.
You can also use the Walmart Gift Card to make purchases of anything in a Walmart Store or online at Walmart.com