Gases like air are weightless?

You can weigh a gas in its gaseous form. Take a strong container and pump all the air out of it, then weigh it. Since there is a vacuum inside the container and a vacuum is weightless, this weight reading is the weight of the container only. Then fill it with the gas and weigh it again, it will be a little heavier.

The difference between the two weights is the weight of the gas

Well they are measured by pressure