The kinetic energy of the hadron's constituents, moving at near the speed of light, contributes greatly to the hadron mass;[1] otherwise most of the rest is actual QCD binding energy, which emerges in a complex way from the potential-like terms in the QCD Lagrangian.
In the standard model, this "quark current mass" can nominally be attributed to the Higgs interaction.
The amount of energy per single gluon, or "gluon energy", cannot be directly measured, though a distribution can by inferred from deep inelastic scattering (DIS) experiments (see ref [4] for an old but still valid introduction.)
Unlike photon energy, which is quantifiable, described by the Planck–Einstein relation and depends on a single variable (the photon's frequency), no simple formula exists for the quantity of energy carried by each gluon.
[3] Currently, the total QCD binding energy per hadron can be estimated through a combination of the factors mentioned.