Most dometic refrigerator have a button you need to push when you shift the mode switch to gas. This button holds the gas valve open until the thermopile generates enough power to hold the valve open. So on most shift the mode switch to gas, hold the button until the flashing light goes out. The flashing light indicates the gas valve is energized and the unit has shifted to gas mode. Depending on when the unit was last used it may take up to 5 minutes for the gas supply to purge the gas lines to the refrigerator. So be patient.
Hope this helps.