  The total gluon helicity in a polarized proton, measurable in high-energy
scattering, is shown to be the large momentum limit of a gauge-invariant but
non-local, frame-dependent gluon spin $\vec{E}\times \vec{A}_\perp$ in QCD.
This opens a door for a non-perturbative calculation of this quantity in
lattice QCD, and also justifies using free-field expressions in the light-cone
gauge as physical observables.
