I recommend:
Gastroenterology - the main journal of the AGA. However it tends to be very technical and is geared more towards researchers.
Clincal Gastrolenterology and Hepatology - the clinical practice journal of the AGA, thus obviously it prints the clinically oriented studies and reviews.
American Journal of Gastroenterology - "the red journal"; the offical journal of the ACG. A good all around journal.
Gut - british GI journal
and lastly, don't forget that the NEJM, Lancet, etc often have the most pertinent discussions in every field.