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It is completely affected by gravity.
However, since it has a low molecular weight, and therefore a lower density, all the heavier gasses get "under" it and force it up in the atmosphere.
Hydrogen is even lighter and will "float" on an atmosphere of helium.
This is all because of gravity that it goes up i.e. away from the center of mass that causes gravity.
In free fall, in a pressurized environment, the lighter gasses like helium are pushed to the outer edge of the environment by the heavier ones.
