This is from before my time, although I've seen a couple of working models. There was a hand pump, and you pumped the gas into the clear container at the top. You calculated how much gas you were purchasing by looking at the gallon markers on the side, then it used a gravity feed to drain into the car's gas tank.
I know a lot of older Freepers know how it works, but some of the younger ones may never have seen one, except one that's been repurposed as a gum-ball machine or aquarium.