In this paper, new upconversion (UC) phosphors for Yb3+–Er3+- and Yb3+–Ho3+-doped Gd9.33(SiO4)6O2 (GSO) were designed via a solid-state reaction method. The phase purity of the samples was examined using XRD patterns. In Yb3+–Er3+ -doped GSO, the characteristic emission peaks of Er3+ appear upon 980 nm excitation, and the optimum Er3+ concentration was found to be 1 mol%. Different changes in the intensity of green and red emissions with Er3+ concentration appeared, and were interpreted by cross-relaxation between Er3+ ions. In Yb3+–Ho3+-doped GSO, three emission peaks of Ho3+ were observed, and the power-dependent UC luminescence was studied. For studying the temperature-sensing properties, different strategies were employed, including thermally coupled levels and non-thermally coupled levels. High sensitivities were obtained in the phosphors, and GSO:Yb3+,Er3+ showed the highest sensitivities. The above investigation results indicated that the developed GSO:Yb3+–Er3+/Ho3+ phosphors could have potential applications in optical thermometry.