What is Bona Gratia?

Legal Definition
In the Roman law. By mutual consent; voluntarily. A term applied to a species of divorce where the parties separated by mutual consent; or where the parties renounced their marital engagements without assigning any cause, or upon mere pretexts. Tayl. Civil Law, 361, 362: Calvin.
-- Black's Law Dictionary
Legal Definition
With good grace; freely.
-- Ballentine's Law Dictionary