The primary reservoir drive mechanisms are (1) Gas drive which includes both solution gas drive as well as gas cap drive (2)Water drive (3) Gravity drainage and (4) Combination drive.

If the gas cap was in fact loose or faulty, just tighten it or replace it and the light will clear itself after a couple drive cycles.

cars i've had that do the 'check gas cap' thing reset automatically after the car is driven and shut down several times. if you drive every day, it should reset in a few days.

Gas Cap is not on tight, or the seal leaks and you need to replace the cap.

find and remove gas cap, fill with gas and replace gas cap.