If a twist on type, turn it counterclockwise to remove.
Scott Drake and likely others offers a kit that adds a locking function to any classic Mustang twist-on gas cap. It is a pin type thing that you need to open the truck to pull the pin back so you can then turn the gas cap. Pretty cool, I just stumbled on it on CJPP this past week ...
Scott Drake ACC-9030-LOCK Mustang Gas Cap Lock 1965-1973| CJ Pony Parts
... maybe your car has one of these !!
Doug