There are many examples of gons, including a fair number off-line, carrying coal, in the Bob Charles Collection that was the fodder for the NMRA Postwar Freight Car Fleet book. Off the top of my head, I can think of NH, B&M, MILW, NYC, and PRR (the NYC is a gon where I can't see the load, but it's in a cut of hoppers).

Cheers,
Ted