Chemical substitution plays a key role in controlling the electronic and magnetic properties of complex materials. For instance, in EuO, carrier doping can induce a spin-polarized metallic state and colossal magnetoresistance, and significantly enhance the Curie temperature. Here, we employ a combination of molecular-beam epitaxy, angle-resolved photoemission spectroscopy, and an effective model calculation to investigate and understand how semilocalized states evolve in lightly electron-doped Eu1-xGdxO above the ferromagnetic Curie temperature. Our studies reveal a characteristic length scale for the spatial extent of the donor wave functions which remains constant as a function of doping, consistent with recent tunneling studies of doped EuO. Our work sheds light on the nature of the semiconductor-to-metal transition in Eu1-xGdxO and should be generally applicable for doped complex oxides.