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Ultrathin Gd films have been studied with constant initial-state spectroscopy (CIS) of photoemission utilizing linearly polarized light of synchrotron radiation. The photoemission cross section of the 5d surface state near EF shows strong photon energy dependence, i.e., sharp peaks in CIS spectra near the Gd 5p1/2 absorption edge. These peaks originate from resonant photoemission processes involving the occupied 5p and unoccupied 5d levels of the surface atoms. The symmetry of the unoccupied states were determined experimentally by changing the polarization of light, where pp-polarized light excites only to the unoccupied dz2 or dxz,yz states and s-polarized light to all the d states including dx2-z2 and dxy states. Based on the symmetry assignments, the lower bound of the exchange splitting of the Gd 5dz2 surface state can be abstracted, which increases when the thickness of Gd film decreases and reach maximum at the monolayer limit.