While the reference mentions methane as "marsh gas," it does not refer to any gas as "smart gas." Therefore, there is no gas widely known or officially designated by that term. It's possible the term is used colloquially or within a specific, limited context.
It's important to note that terms and nicknames for chemical substances can vary and may not be universally recognized. Without additional context or a more specific source, identifying a gas definitively called "smart gas" is impossible.
