This question is based on the FDA's Federal Food, Drug, and Cosmetic Act, where they list medical gases as being regulated by the FDA. There is ambiguity, however, as the act puts an emphasis on high purity gases and does not appear to mention the regulations on these same gases at lower purities or in mixtures with other gases.
This is particularly confusing when it comes to oxygen and nitrous oxide, as both are listed as medical gases, with oxygen is being sold legally both as a supplement in a can as well as at "oxygen bars", just at a purity lower than what would classify it as "medical oxygen". Nitrous oxide, however, cannot legally be sold for human consumption in this same way. Regular air is also listed as a medical gas, and presumably this could be sold legally like oxygen as long as it's not for medical purposes.
Additionally, what do this act or other laws say about gases not explicitly listed as medical gases? Can a gas even be sold as a supplement in the first place?