Can you actually present an image where its said to be gravity related?
Because the Chibaku Tensei has been show many times not to drag people towards it, it drags the Earth around them which pushes people into it.
Just look at the fight with Nagato, it was the rocks that were pushing them into it, and during the time Madara used it, again it didn't drag anyone near by into it.
The idea that it is gravity related was something fans speculated on, because the ability itself has never been explained.