The 2015 Buick Verano doesn’t have any switches, buttons, or levers inside the cabin, so your plight is understandable! To open the gas tank on a 2015 Buick Verano, push on the left-hand side of the fuel filler door on the passenger’s side of the gas tank.
The hinge to the fuel filler door is spring-loaded, so when you push it in, it unlatches and opens. Once you’re done refueling, push the door in the same manner to lock it in place.