The gas cap message could also mean Large Leak detected.
Look at around gas tank where all the lines connect into the tank & look for any cracks in lines, vent tube lines, filler neck etc.
Car running without any problem is now China is very hot, but there is no gasoline in the vicinity of the car should be a small amount of leakage, I do not know how you are looking for the leak? Gas cap is bought on eBay