Gesundheit is wishing good health. This is what I use if I say something. And in many instances it is polite/expected to say something.
I used to say God Bless You when I was religious. Then I stopped being religious and said Bless You. Then someone responded to me "Yes, he did" and I found that creepy and stopped saying anything. Then that was sometimes uncomfortable and/or came off cold so I started saying Gesundheit.
If you look at the Wikipedia list of "Responses to sneezing" it seems like half of them are some variant of "to your health!" and most of the rest are variants of "God bless you".
There are various origin stories for "Bless you" but most of them seem to deal with trying to use the magic of God to fight disease which strikes me as backwards and not to be encouraged. Wishing someone good health is just as ineffective but less backwards. If you don't like German you can use Spanish "salud!" or even just English, saying Good Health! or To Your Health! or Fast Recovery!