If someone is sneezing you have to say "salud" or "Jesus". Jesus if you are a kind of religious person and salud if you aren't this kind of person. But if you mean "God bless you" in Spanish would be "Que Dios te bendiga" Many years ago grandparents used to say "con Dios" instead of goodbye. In fact some of this old people still saying this expression "Con Dios" something in English like "With god" or "Go with God"

"Salud" is the most common thing to say after someone sneezes. It literally means "health". If you actually want to say "bless you", it's "Dios te bendiga". It means "may God bless you." It's a longer phrase, but it is also very much said.

It's an expression of well-wishes: - bless you = ¡Jesús! - bless you = when you're sneezing, someone says to you: ¡salud! - God bless you = Christian expression of well-wishes = que Dios te bendiga.

Se suele usar más "salud" en España, pero también "jesús" está bien.