An analogy might help here:
Joe takes out comprehensive insurance on his car but omits to declare that he's modified it by adding a sports spoiler to it. The car is later destroyed by fire (which has absolutely nothing to do with the presence of he spoiler). An insurance assessor inspects the wreckage and spots the remains of the spoiler. The insurance company has the right to (and almost certainly would) refuse to pay out a single penny.
Similarly, if you need to be hospitalised and repatriated by air ambulance after (say) a heart attack and the insurer calls for a medical report (which mentions your undeclared gastric problem) the insurer would almost certainly refuse to pay out.
Declare the problem and see if you can get cover anyway. If not, contact this specialist broker:
http://www.able2travel.com/