"Gen" is an abbreviation for General Information. I don't know if you still have them these days, but we always had notice boards divided up as "Station Routine Orders" (SROs) and "General Information". Pukka is, as someone pointed out, from Imperial days in India and Duff is short for defective. Pukka Gen was thus stuff that you found posted on the notice board and therefore official (though not necessarily true) Duff Gen was misinformation circulating on the rumour mill.