How do you say gods grace in latin?

The usual expression is Dominus vobiscum (May the Lord be with you).

It is possible to say Gratia vobiscum (May grace be with you) - but this sounds very odd. Grace is only one aspect of God, if I heard gratia vobiscum I would ask myself why the blesser was being antsy about giving the full blessing.

Dominus tibicum (and I suppose gratia tibicum) is also possible, if you are speaking to a single person whom you know very well.