We report on the magneto-optical (MO) properties of heavily Tb31-doped GeO2-B2O3-Al2O3-Ga2O3 glasses towards fiber-integrated paramagnetic MO devices. For a Tb31 ion concentration of up to 9.7 3 1021 cm23, the reported glass exhibits an absolute negative Faraday rotation of ,120 rad/T/m at 632.8 nm. The optimum spectral ratio between Verdet constant and light transmittance over the spectral window of 400– 1500 nm is found for a Tb3+ concentration of ~6.5 x 10^21/ cm³. For this glass, the crystallization stability, expressed as the difference between glass transition temperature and onset temperature of melt crystallization exceeds 100 K, which is a prerequisite for fiber drawing. In addition, a high activation energy of crystallization is achieved at this composition. Optical absorption occurs in the NUV and blue spectral region, accompanied by Tb3+ photoluminescence. In the heavily doped materials, a UV/blue-to-green photo-conversion gain of ,43% is achieved. The lifetime of photoluminescence is ~2.2 ms at a stimulated emission cross-section sigmaem of ~1.1 x 10^-21 cm² for ~ 5.0 x 10^21 /cm³ Tb3+. This results in an optical gain parameter sigmaem*tau of ~2.5 x 10^-24 cm²s, what could be of interest for implementation of a Tb3+ fiber laser.