Effective functionalization of two-dimensional (2D) materials is an important step in order to open new intriguing functionalities. In this work, the magnetism engineering of Ga2SeTe Janus monolayer through doping with transition metals (TMs = Mn and Fe) is studied using first-principles calculations. Pristine monolayer is a direct-gap semiconductor with band gap of 1.21 eV, being intrinsically nonmagnetic. Mn and Fe impurities induce significant magnetism with overall magnetic moments of 4.00 and 5.00 𝜇𝐵, respectively, being produced mainly by transition metals. Moreover, our calculations assert the in-plane magnetic anisotropy (IMA) of Mn-doped systems, while the perpendicular magnetic nisotropy (PMA) is found for Fe-doped systems. Herein, stronger magnetic anisotropy is obtained when TM atoms are doped at Ga2 (Ga atom bound to Te atom) sublattice. Magnetic semiconductor nature is obtained when realizing the Mn doping at Ga1 (Ga atom bound to Se atom) sublattice, while the monolayer becomes half-metallic when considering Ga2 doping site. Otherwise, Fe doping makes new 2D magnetic semiconductor materials regardless the doping site. Further, ferrimagnetic ordering is achieved by doping with pair Mn atoms, while the substitution of pair Fe impurities induces ferromagnetic ordering with very high Curie temperature of 2207.60 K. In both cases, strong PMA with magnetic anisotropy energy up to 7312.56 μeV is confirmed. Our results may introduce new 2D magnetic materials obtained from the doping-induced magnetization of nonmagnetic Ga2SeTe Janus monolayer, where the development of doping-site- and doping-level-dependent featurerich electronic and magnetic properties suggests selective applications towards magnetic field sensing and magnetoresistive random access memory (MRAM) fabrication.