A more thorough explanation:

Definition: Gratia mandatarii is a Latin term that means "for the sake of the mandatory." It refers to the irrevocability of a mandate given solely for the benefit of the mandatory.

For example, a mandate contained in the registration clause of a deed, whereby the granter gives authority for its registration, is not revocable. This is because the mandate is granted solely for the sake or advantage of the mandatory, and not for the benefit of the mandant.

In simpler terms, gratia mandatarii means that if someone gives a mandate solely for the benefit of another person, they cannot take it back once it has been given.