My odometer display has a message saying gas cap. I looked in the manual and it says gas cap fault. I am trying to find out what needs to be done for the message to disappear. I am afraid something serious might be wrong with the car

 

Remove the cap and reinstall. It's probably not sealing correctly. If the message persists replace the gas cap. If it still persists have a shop find where the leak is in the EVAP system. This is not at all serious unless you are the environmental type. It means a small amount of raw gas fumes are being released into the atmosphere instead of being trapped in the fuel system as they should be.