We investigated the physicochemical properties, structure and optical properties of the Ge₂₅Ga_9.5Sb_0.5S₆₅: Er³⁺/Yb³⁺ glasses. The Judd-Ofelt theory was used to calculate the intensities of the intra-4f electronic transitions of Er³⁺ ions. We observed the upconversion photoluminescence (UCPL) at 530, 550, 660 and 810 nm under 980 nm excitation. In the Ge₂₅Ga_9.5Sb_0.5S₆₅: 0.1 at.% Er³⁺, we found that the Stokes photoluminescence (PL) at the green spectral region excited by the 490 and 532 nm laser is only ≈5 times higher than the UCPL emission under 810 or 980 nm excitation making these materials attractive for UCPL applications. The addition of 0.1-1 at.% of Yb³⁺ into Ge₂₅Ga_9.5Sb_0.5S₆₅: 0.1 at.% Er³⁺ glass reduces the UCPL as well as the Er³⁺ ≈1.5 μm emission intensity probably due to the reabsorption processes of the excitation light and concentration quenching. However, the observed Er³⁺: ⁴S_3/2→⁴I_13/2 (≈850 nm) emission in the Ge₂₅Ga_9.5Sb_0.5S₆₅: 0.1 at.% Er³⁺ sample populates the ⁴I_13/2 level, which promises the using of this material for the 1.5 μm optical amplification.