Chalcogenide glasses appear as good candidates to build all optical gas sensors due to their wide infrared transparency and the possibility of incorporate rare earth active in MWIR spectral range. To detect and quantify gases, one way is to develop chalcogenide glasses presenting transparency compatible with the molecules absorption band frequency. Two domains of interest can be distinguished: MWIR and LWIR corresponding respectively to the 3–5 μm and 8–12 μm spectral ranges. Selenide and sulfide based chalcogenide glasses are known for their excellent infrared transmission properties in the 1-15 µm region with good thermo-mechanical properties. Doped with Dy3+or Pr3+, sulfide glass fibers have been used as MWIR source for gas sensor for CO2 detection. To probe the far infrared beyond 12 µm, telluride chalcogenide glasses appear as a very interesting material due to it low phonon energy and a broad transparency (up to 25 µm). While these attractive optical properties of telluride glasses, particularly for LWIR, there is few study about rare earth incorporation for luminescence explained a challenging synthesis process avoiding crystallization. To get more stability in the glass state it is essential to add selenium. Thus for each system, it is required to determine the best compromise between the transparency domain and the glass state stability by playing on the ratio between selenium and tellurium atoms. Regarding the energy level of Tb3+, we can expect to have a radiative emission from 3.1 µm up to 8 µm. For gas sensor application, it is a range of interest regarding the LWIR absorption band of some hazardous gases. Thus, Tb3+ doped chalcogenide glasses with nominal composition of Ga5Ge20Sb10Se(65-x)Tex (x = 0, 10, 20, 25, 30, 32.5, 35, 37.5) were synthetized. Their physico-chemical properties (chemical composition, density, thermal characteristics) and optical properties (transmission and ellipsometry spectroscopies) are clearly modified by tellurium substitution to selenium. Based on a detailed study of the Ga5Ge20Sb10Se(65-x)Tex bulk glass and fiber properties, the optimal composition of seleno-telluride glass fiber was found to be Ga5Ge20Sb10Se45Te20. The luminescence properties of Tb3+ (500, 1000 and 1500 ppm) doped Ga5Ge20Sb10Se65 and Ga5Ge20Sb10Se45Te20 were studied in glass bulk and fiber samples. Radiative transitions calculated from Judd-Ofelt (J-O) theory were compared to the experimental values. Although an expected lower phonon energy for telluride glasses, selenide glasses stay more suitable for MWIR emission with a strong emission at 4.8 and 3.1 µm. The emission at 8 µm was successfully observed with careful luminescence investigations.