When sneeze, people will say God bless you. What one should reply?

Either a "Thank You" or a personal response.

"Zeus thanks you" is an appropriate response in most situations.

The responce to smeone sneezing comes from the black death (plague) in the medievil age. The iden was that when you sneeezed that was an oppertunity fo satan or an evil spirit to posess etc the person. Therefore thats why people say bless you or god bless you to be more specific. Mosy people dont relise this responce and where it comes from we just do it.