Gravity center = barycenter = average point.
And if you use 'Edit > Octree > Resample', CC will compute the 'gravity center' (it creates a new point that didn't exist before, hence the 'resample' formulation).
And if you use 'Edit > Subsample' and you use the 'Octree' mode, CC will take the point closest to the octree cell center (no new point is created, hence the 'subsample' formulation.
About OCtree
Re: About OCtree
Daniel, CloudCompare admin