We argue that inclusive direct photon production at large transverse momentum in proton-proton interactions with longitudinally polarized beam and target is an incisive probe of the polarized gluon distribution in a proton. We provide predictions of cross sections of range of reasonable choices of the polarized gluon distribution. At current fixed target energies, the cross sections are small but measurable. At collider energies, such as the SSC energy, isolated direct photon production should be considered in order to minimize the nonperturbative contribution due to fragmentation. 9 refs., 3 figs.