What gas has 8 protons and 8 neutrons?
Well, we use the number of protons and number of neutrons and add them up to give us the average atomic mass.
Therefore the average atomic mass is
In addition, at room temperature, oxygen's state is gaseous.
Now there is something called "isotopes", in which the atom of the same element, has an altered configuration - different number of neutrons. O-16 is the most common isotope of oxygen.
Thus, we can conclude that the gas you are specifying is oxygen. To be specific, oxygen-16.
Hope this helps :)