My nan grandmother (his daughter) told me about this when I was little. she gave me the impression it was about on par with stealing pens from work
Serious. Penalties for theft or fraud of ration books was fine of £500 and a possible two years in prison. Might have been more severe for your GGF as he was in uniform and subject to military justice, not civilian. But it would not be the equivalent of stealing pens as the system was designed to try and ensure each Briton would be adequately and equally fed during the war. That being said, it appears that fraud and theft was rather rampant.
Austerity in Britain Rationing, Controls, and Consumption, 1939-1955, Ina Zweiniger-Bargielowsk
Black Market Britain: 1939-1955, Mark Roodhouse
EDIT: This comes from my personal reading as a result of my studies in law school. A more in depth historian will hopefully be able to provide more clarity and color, as WWII historians are not in short supply in this group.