Earths gravity pulls every object. Nothing can escape gravity. Gravity is the force which keeps one tied to the earth. Law of Karma operates in the earthly plane. Nobody can escape the law of karma. To escape the gravity, rockets launched from earth need to carry enough fuel to provide the initial thrust. Similarly, to escape from the law of karma, thrust of Sadhana - Spiritual practice is required. Once the rocket escapes gravitational field, rocket can glide smoothly in the space. Similarly, after the Sadhana provides the initial thrust, the spiritual aspirant can effortlessly continue Sadhana. Now the Law of Karma will have very less effect on him/her. Force of Grace will start operating on him/her. Grace is the Mysterious force of the Lord. Without Grace nobody can succeed in Spiritual practices. While Sadhana is ones own effort, Grace is the gift of the Lord. Grace flows when there is Sadhana and grattitude to the Lord. Grace is a mysterious force which provides the appropriate conditions for one to proceed in the spiritual path, provides the right inputs on the path at right time, brings one to Sadguru - enlightened Master and helps one to See the Truth.
Grace is quite opposite of that of Gravity. While Gravity keeps one earthbound, Grace liberates.