In this paper, the effects of hot isostatic pressing (HIP) temperature on the crystal structure, optical, dielectric and magnetic properties of GdMnO3 ceramics were studied. All samples form a single-phase structure without structural transformation, while HIP temperature induces the changes in lattice parameters. HIP causes the change in Mn ions valence state, oxygen vacancy concentration, Raman vibration modes and microscopic morphology of GdMnO3. Vacancy concentration of the samples prepared by HIP at 800 °C increases compared with that of the samples without HIP, then remains unchanged when the HIP temperature is from 800 to 900 °C, and finally decreases with the further increase of HIP temperature. Appropriate HIP temperature can increase the dielectric constant and decrease the dielectric loss. The transition temperature of paramagnetism to antiferromagnetism and magnetization can be significantly affected by HIP temperature. Magnetic transition temperature and magnetization are closely related to Mn2+ ions concentration and cation vacancy concentration, respectively.