The role of Sm doping on the structure, vacancy characteristics, valence states of the constituent elements, and magnetic performances of Gd1-xSmxMnO3 (x = 0.00–0.20) ceramics synthesized using the solid-state reaction method is discussed. The X-ray diffraction and Raman spectroscopy results revealed that the orthorhombic perovskite structure with a single phase was formed in all Gd1-xSmxMnO3 ceramics, and the addition of Sm ions induced lattice deformation. The X-ray photoelectron spectroscopy and positron annihilation spectroscopy results showed that the vacancy concentration and Mn4+ content in Gd1-xSmxMnO3 samples initially increased and subsequently decreased as the Sm doping concentration was increased. The magnetic property measurements revealed that Sm doping had a significant impact on the low temperature magnetic ordering of Gd3+ ions, magnetic transition temperatures, and magnetization. From the correlation study between structure and properties, it could be concluded that the development of magnetic characteristics was closely related to the cation vacancies concentration and the creation of Mn4+ in Gd1-xSmxMnO3 samples.