Quoting from the article (which quotes from the paper): "Note that I am not even attempting to find an analogy for the gluonic gravitational form factors that

Quoting from the article (which quotes from the paper): "Note that I am not even attempting to find an analogy for the gluonic gravitational form factors that would help you understand them. They're described in the paper as "the matrix elements of the energy–momentum tensor of the proton" The energy momentum tensor is the same as the stress energy tensor.