Which of these is not a gas? hydrogen, oxygen, gravity, or helium

Answer: Gravity.

Explanation: Gravity is what holds the earth's atmosphere to it (which is a mixture of gases). For example, Moon doesn't have an atmosphere only because of a weak gravitational field but that doesn't mean that gravity can be considered as a gas.

Other options (Oxygen, Hydrogen and Helium) are all gases.

I think that Gravity is not considered a gas but it can at the same time

Explanation:

Gasses are affected by gravity, as above video will prove ... Yes, there is a attractive force between the gas molecules and gravity!

The rest are for sure gases