Generic reference to absolutely any convenience store, anywhere in the world regardless of local chains' ownership or predominance.
The store may actually be a Circle K, an AM-PM, an Amoco, or an Esso, but to a road-weary traveler facing the 12th of a 32 hour road trip, through an unknown state or municipality, at highway speed, they're universally referred to as "Gas-N-Sip."
The name is derived from the movie "Say Anything," wherein one character questions the mental/emotional superiority of a group of youths who spend all their free time loitering about the parking lot of a nondescript convenience store named Gas-N-Sip.
"Dude, it's gonna be a long haul crossing the Badlands. Better hit the next Gas-N-Sip so we can replenish."