Some are ... some aren't. If the pump lights are lit, the pump is most likely active - however, if the station is dark and the parking lot lights are out ... do you really want to be pumping gas in the dark?
When I travel, I usually have no problem finding stations that are open all hours of the night ... often these are convenience stores - especially name brands like 7/11, Pilot, Wawa, Love's, RaceTrac and RaceWay and many Walmart stores have a Murphy station.
Even without an app like GasBuddy - you should be able to ask Google or Siri or Alexa or whoever your device has something like "24 hour gas station near me" and your device should give you some results ... even help with directions.
I just tried it on my Android phone and was given several stations that all show "open 24 hours" in the results