Act 1 scene 2
In this private scene with the Duchess of Gloucester, Gaunt acknowledges that he knows who has killed the Duke, but since it is the king himself, and the king is the one who should be avenging the murder of any of his subjects and is not doing so, and since he has sworn fidelity to Richard as King, Gaunt feels that God should be the avenger. This view that God will right all wrongs is very representative of the Renaissance Christian Humanist belief structure and marks Gaunt as a Humanist.
The Duchess of Gloucester's argument is that since the blood of the same father is in both brothers, Gaunt is not protesting his own murder by staying quiet and not protesting against Gloucester's murder.