I have a simple apparatus: a box and inside it a test particle (small bullet). If the device is in inertial motion or affected only but (static) gravitational field of another massive body (in free fall to the Earth, on orbit around Sun etc.), the test particle doesn't move (or moves with constant velocity) relative to the box.
I assume that if a gravitational wave is passing through the box, I can't detect the wave with such a apparatus. In other words, the motion of the test particle relative the box is unaffected by the wave. Am I right?