Yes, it's true that gravity would count as an outside force. Consider the law (which is in fact a formal version of the law you mentioned) that Force = the product of mass and acceleration i.e. F=ma. Re-arrange this equation to get a=(F/m). Now imagine that a small object is being attracted towards a larger object in a vacuum (i.e. there is no air, and no other substance to slow it down.) If gravity were not an outside force, this would mean that there would be no forces at all acting on the small object. You might try to argue that an inside force can still affect the movement of the object and hence is considered for the value of F, but if you consider the force as a vector (a number that has magnitude and direction), you will see that the fact that it moves the object means that it can be considered as acting through, rather than inside, the object without making any difference to the mathematical model (because location is not included in the definition of a vector.) In other words, any force moving the object must either be a push or a pull, so it can be represented as acting outside the object and affecting the whole of its motion, which is what you might say is happening in Newton's system if you apply the idea of emitting gravitational waves. So in this situation F=0, so whatever value m takes, a=0, but you can imagine that in space a rock or satellite approaching Earth would pick up speed, rather than move at a steady rate. Even if you visualise an object inside Earth's atmosphere, you know it accelerates towards the ground, but the situation of gravity being an inside force means air resistance is the only force acting on it, so, contrary to common sense, it would slow down. I hope that gives you a picture that you can treasure of why gravity must be an outside force.
To be honest, though, I was more interested on the question you posted on Nietzsche, but was too late to answer. I have extensive knowledge on the subject, so please feel free to e-mail me on my yahoo account with any further questions on Nietzsche that may crop up when reading his works.