Saying "Bless you" after someone sneezes can be problematic, and not just for licensed ministers. Being a member of the clergy has taught me that many Americans are simply uncomfortable with their spirituality. So uncomfortable that they simply don't want to hear, "Bless you", even if it means their soul might escape through the nose.
"Gesundheit" is a secular expression, but it's in German. It's hard to sound comforting in German. After watching a dubbed version of Titanic, and hearing the faux-Dicaprio seem to shout at the faux-Winslet, my German-speaking friend concluded that it was impossible to sound loving while speaking German. Even saying, "Good health", sounds like an order. "GesundHEIT!" Maybe you can scare an escaping soul back to the body?
Ultimately, the sentiment we're all trying to express is, "Aww, you sneezed." Why not go with that? And if you still want to keep it spiritual, try, "Oh God, you sneezed!"