  The near threshold photo or electroproduction of heavy vector quarkonium off
the proton is studied in quantum chromodynamics. Similar to the high-energy
limit, the production amplitude can be factorized in terms of gluonic
Generalized Parton Distributions and the quarkonium distribution amplitude. At
the threshold, the threshold kinematics has a large skewness parameter $\xi$,
leading to the dominance of the spin-2 contribution over higher-spin twist-2
operators. Thus threshold production data are useful to extract the gluonic
gravitational form factors, allowing studying the gluonic contributions to the
quantum anomalous energy, mass radius, spin and mechanical pressure in the
proton. We use the recent GlueX data on the $J/\psi$ photoproduction to
illustrate the potential physics impact from the high-precision data from
future JLab 12 GeV and EIC physics program.
