Gas is signed by mimicking a fuel pump nozzle with your dominant hand and the hole of your car's gas tank with your non-dominant hand. First, form your non-dominant hand into a loose fist or ASL letter S sign. Next, form your dominant hand into a loose ASL letter A sign, with the thumb sticking out. Stick the thumb of your 'A' hand into the opening of your 'S' hand.
Teach your toddler the gas sign when you drive up to a gas station and need to fill up your car's gas tank. You can also explain that this gas is what your car, truck, and most other vehicles need in order to run.
Teach your toddler this sign in conjunction with an actual visit to the gas station, or when they ask you what the fuel dispenser or gas tank is whenever they see it in pictures or cartoons.