We investigate the photoluminescence (PL) polarization of millimeter-scale GaSe films with 100 nm and 150 μm thicknesses by excitation with linearly and circularly polarized light. Although free exciton emission predominates at 140 K, multiple peaks appear at 30 K owing to the recombination with impurity and/or localized states. Despite the circularly and linearly polarized excitations, the 100-nm-thick GaSe film constantly shows linear polarization in both band-edge and impurity/localized PLs. By rotating the sample geometry, the observed linear polarization is also rotated. This indicates that linear polarization is sensitive to the sample geometry and originates from the optical waveguide effect.