Walmart says that the funds do not expire on the Walmart Gift card but then they say that you cannot use the card after the expiration date which is usually 5 years after purchase date.
So technically the card does expire and you may not be able to access the funds from the Walmart Gift Card.
So as long as you use the Walmart Gift Card within 5 years and use the funds up on the Walmart Gift Card within 5 years then you should not lose the funds.