There are definitely diesel G Wagons. The last several months of the Star magazine (MB Club) have had an ongoing serial feature article of a guy who drove his diesel G Wagon from the US down to Tierra del Fuego and back. Very interesting series.

I don't remember the specifics of his G Wagon, but I think it was a late 80's vintage. The only engine trouble he had on the whole trip was some fuel gelling up in the Andes somewhere, preventing starting one day.

Rgds,
Chris W.
'95 E300D