As long as you're of legal age to buy the beer you can use the Walmart Gift Card to purchase beer at Walmart Stores.
You can purchase anything at Walmart that you want to buy with the Walmart Gift Card because Walmart Gift cards are just like using a debit or credit card but the only difference is you can only use the Walmart Gift cards at Walmart locations.
You can't use the Walmart Gift Cards at other stores other than Walmart so if you want to purchase beer at Walmart with the Walmart Gift card you can do so as long as you're of legal age to buy the beer.