Is the volume of a gas always the volume of its container?
Yes, if contained.
Yes, only if the gas is contained and is purely a specific gas.
This is because gas has the property of having an indefinite volume and shape, however, if we can contain the gas into some sort of container - like a balloon - then we calculate the volume of the gas by calculate the volume of the container.
Keep in mind that in the balloon's case, the volume changes often, due to the elasticity of the material.
Hope this helps :)