They are gerunds.
The sentence is typical medical jargon, and would be perfectly intelligible to doctors, nurses and scientists.
More than one-half of gastric ulcers occur in males, and they are less common than duodenal ulcers. (It's not the one-half that are less common.) However, this lower rate of occurrence may be at least partly an illusion due to the fact that gastric ulcers often have no symptoms and are discovered only after a complication develops.