Since almost all of the mass of individual atoms is in their nucleus, gravity must be a nuclear force.
Are you saying that it is exclusively a nuclear force, or merely a predominantly nuclear force?
Of course, the real problem is that because gravity is such a week force, we really have little idea of how gravity behaves at sub-atomic levels.
The fraction that is nuclear is the fraction of the mass that lies in the nucleus, ~99.9%.
Calculate the force of gravity at sub-nuclear distances, <10^-13 cm, to see if it is "weak."
With kind regards,
Oliver K. Manuel