Gravitational mass is the property of objects that determines how they interact via gravity. For example how the Moon rotates around Earth. Inertial mass is an object's property that determines how much the object "resists" acceleration when force is applied to it. And it seems both are one and the same property. That means if we had the Moon in the void of space and we had some reference point relative to which the Moon did not move, then we applied force to the Moon and measured how it accelerated then we could tell exactly how would it rotate around Earth, for example how fast it would have to rotate at given distance in order for its orbit to be circular. What seems to me as a logical question is: Why are those two properties the same? And I don't mean an answer like "because GR sais so". I may be wrong, but I think GR postulates them equal, it doesn't explain why.