For the Gravity Mass Array, feed it a list of Gravity Mass Components (not the actor) that you want to affect this instances OrbitMovement Component. If left blank, it will default to all GravityMass objects in the scene. This is only used in Kinematic mode.
To select the largest influence, you will need to loop through every Gravity Mass, check the gravity acceleration exerted by using the Gravity Mass “Gravity Acceleration Exerted” function, and send whatever has the largest value to the Gravity Mass Array. There is an example of how to do this in the SetGravityMassOrbitingToLargestInfluence function inside BP_Orbit (that function is used by default when using Kepler physics mode).
For version 4.18, I will add a function called something like GetLargestGravityMassInfluence to make this a bit easier.