They say you can use them with the Walmart app now. The system seems to work similar to the other gas station apps.
https://thepointsguy.com/news/walmart-plus-exxon-mobil/
I'll be trying it soon. My family was targeted by AMEX for a lot of those Walmart AMEX offers, so I've picked up several discounted Walmart gift cards. I used them for gas on a recent roadtrip iin the Southeast, but there was certainly a bit of a learning curve in finding (and differentiating) between Walmart's own fueling stations, Sams Club and the two different Murphy brands. Things work differently at each. My pro tip is to load your Walmart gift cards in advance to their app, but also retain the GC in case you need it elsewhere.