Classifying gas as a sustainable investment is defensible, albeit wrong.

Gas is good in the short-term for stabilizing a power supply that doesn't yet have enough storage. Coal is too slow to respond, hydro is limited by geography, and batteries are on the cusp of major advancements that may render them obsolete in the near future (especially at grid scale). Furthermore, as I understand it gas equipment can be used to burn stored hydrogen as a kind of battery storage. In the future perhaps that could be a use case for the decades this gas infrastructure will need to be used for to be profitable.

If it were up to me I DEFINITELY would not classify it as green though.