The gas material represents a single pure gas substance. It is managed by the Gas model as outlined in the section describing single-component material models. By default the Gas model takes the gas material air from the material database.
The screenshot below shows the Gas model node and its child node Air, which represents the pure gas material (air) being modeled.
If you wish to model a gas other than air you can replace the existing gas material with a different one from the material database, as outlined in the section describing how to manage single-component materials. Alternatively, you can edit the properties of the gas directly as outlined in the section on setting material properties.