Although Gerasim is sure of the fact that he and everyone shall die someday he maintains an absolute equanimity.
That description of him: "The even white teeth of a healthy peasant", emphasized by the author narrating the dialogue between Gerasim and Ivan, who was laying on his deathbed, allows us to imagine a person who has such peace of mind and soul, that he is not worried about death, therefore, while still alive, he takes good care of his health, a fact represented by his "even white teeth". Gerasim is trying to provide peace to Ivan, and he is under such remarkable serenity that he even smiles a bit, thus, allowing "us" to see his white teeth.