"G"'s were supposedly all built with two tanks, although Deere marketed gasd pistons and manifolds for them to be installed in the field. I've also read these parts were not available until G production has ceased in 1953.
The last I knew, there had been two G's found that had no evidence of ever having a starting tank for all fuel, supposedly meant for gas right from the factory. No price list ever showed gas as an option for the G, so who knows how many were built, probably only a handful. Perhaps more have been found in the past decade or so?? But you MAY have stumbled on to something quite rare!
Please post a picture or two!