Unless you think that Gravity can predict *where* the Sun and the Earth will be in the future, then Gravity must pull towards the center of the mass that originated it... where that center of mass was located when the Gravity wave first propagated away from that center of mass.
If the Sun didn't move, then this would yield us no useful information.
But the Sun does move, in fact it is moving rapidly (157 miles per second).
Light, for instance, takes 500 seconds to travel from the Sun to the Earth. In that 8.3 minutes, the Sun will have moved 78,186 miles. Thus, when you "see" the Sun, you are actually seeing where the Sun was located 8.3 minutes ago.
...And if Gravity travels as slow as Light, then the Earth is going to be orbiting in a plane that is centered around where the Sun was located 8.3 minutes ago, too.
And we *can* measure and observe the angle and center of the plane of the Earth's orbit around the Sun, FYI...