Simulating gravitational attraction between bodies - follow up
Disclaimer: I have no idea about Unity3D.  

that being said, if there is no reason regarding Unity3D to keep this public


public float gravityConstant = 1;
public Vector3 startingForce;  



then make it private.  



I don't like this method writing style  


public void Start()
{ this.rigidbody.AddForce(this.startingForce); }

public void FixedUpdate()
{ this.ApplyAttraction(); }  



IMO if something doesn't fit good into the same line, one should use 4 lines of code.  



The only obvious place where you can gain some performance boost would be at calculating the attraction. 


float attraction = 
              (this.gravityConstant * this.rigidbody.mass * body.rigidbody.mass)
            / (Mathf.Pow(force.magnitude, 2));  



instead of calling Mathf.Pow(force.magnitude, 2) a simple force.magnitude * force.magnitude will be faster, because there won't be done any checks if e.g the power contains a valid value.