Burping is rude, but flatulence is natural. Go with it...
I share the same problem as you in that I need a couple of hours to digest my food properly. Training earlier then that will cause some discomfort ranging from your flatulence problem to real pain and rolling in the stomach.
My solution... eat two hours or earlier before training, or eat afterwards. If normal ppl can fast the whole day without dying, a couple of hours training before dinner won't kill you either.