I had a dog go off food for several days, then his appetite returned. NO diarrhea, no vomiting, just lack of appetite and a bit grumpy. A month later, again, no appetite for 5 days. By day 5 I had bloodwork run and he was in kidney failure. He spent the next two weeks at MSU being treated for leptospirosis.

IF you haven't had a complete blood panel run and other tests, I would recommend it. Most dogs do not go off food for no reason.
Sometimes the organs are not working properly which affects the appetite. You don't know exactly what is going on inside without running some tests.

I would recommend giving digestive enzymes, probiotics and try to do these naturally(green tripe suggestion is what I feed for these, as well as milk kefir for more probiotics)
Your dogs gut must be a bit out of whack if there is gas happening....probiotics may help balance the flora.
I hope it is just 'picky' eater and nothing more serious!
SweetClover3.17.94~11.24.08 Kacie 7.21.2005-5.01.2015