Since the force of gravity is K*M1*M2/d^2, it is the sum of the gravities of every other body in the universe, so starting closest to Ganymede but becoming negligent once Jupiter, Saturn and the Sun are factored in. If it is not close and not heavy, it is negligent. https://en.wikipedia.org/wiki/Planetary_mass The value varies over time based on the orbits of all the other bodies and Ganymede, especially other moons of Jupiter. The irregularity in its own orbit is a substantial factor: https://en.wikipedia.org/wiki/Ganymede_(moon)#Orbit_and_rotation
