。
She died of stomach cancer.
Literal
She [topic-は] stomach-cancer [from-で] died.
Cause-of-death で is the standard frame: noun + で + 死ぬ. The same で marks cause/reason in 風邪で休む ('miss work due to a cold'), 事故で怪我する ('be injured in an accident'). 胃がん is among the more common cancers in Japan — historically high incidence, partly attributed to high-salt diets and prevalence of H. pylori infection. The blunt 死んだ ('died') is direct; in more polite or formal writing, 亡くなった ('passed away') replaces it.