Gastric cancer is common, but not strongly correlated with lifestyle/diet. Several dietary factors are mentioned, but because diet is strongly influenced by ethnicity, and ethnicity by genetics, it's hard to really evaluate these things.
Check the first section here on "Etiology"
https://www.ncbi.nlm.nih.gov/books/NBK459142/ As you can see, most things it's assoicatied with are out of your control.