Germanium selenide nanoplates (GeSe NPs) are considered to have broadband nonlinear optical (NLO) properties and great potential for applications in nanophotonic devices. In this work, we systematically studied the NLO response of GeSe NPs by the open-aperture Z-scan technique. GeSe NPs exhibit strong saturable absorption at wavelengths of 400, 800, and 1064 nm with different pulse durations. Furthermore, we investigated the excited carrier dynamics of GeSe NPs by the non-degenerate pump-probe technique. The fast and slow relaxation times at different wavelengths of 800, 871, 1064, and 1100 nm were components with lifetimes of about 0.54–1.08 and 52.4–167.2 ps, respectively. The significant ultrafast NLO properties of GeSe NPs imply their potential in the development of nanophotonic devices. Here, we designed and fabricated the all-optical diode by means of the GeSe/C 60 tandem structure and demonstrated that the saturable absorption behavior of GeSe NPs can be used to fabricate a photonic diode, which exhibits nonreciprocal transmission of light similar to that of an electron diode.