We investigate the structural, magnetic, electronic and optical properties of a transition metal-doped GaTeCl monolayer, denoted as M@GaTeCl (M = V, Cr, Mn, Fe and Co), by using first-principles calculations. It is found that the magnetic ground state can be regulated by different M elements. In the meantime, the electronic structure is different with the doping of different M metal atoms, and thus the optical absorption changes correspondingly. The electronic calculations of M@GaTeCl suggest that V@GaTeCl, Cr@GaTeCl, Mn@GaTeCl and Fe@GaTeCl are semiconductors and the magnetic ground states are G-type antiferromagnetic (AFM), C-type AFM, A-type AFM and C-type AFM order, respectively, while Co@GaTeCl is a metal and the ground state is ferromagnetic (FM) order. The different magnetic ground states are discussed with the Heisenberg model. The rough estimation of the ferroelectric polarization value of M@GaTeCl suggests that M@GaTeCl still exhibits multiferroicity. The electronic structure is explained by the projected density of states, band structure and decomposed charge of the valence band maximum (VBM) and conduction band minimum (CBM). Simultaneously, the absorption coefficient calculations indicate that M@GaTeCl absorption shows anisotropic properties, as the same as in a pure GaTeCl monolayer, there exists enhanced visible light absorption in these M@GaTeCl monolayers relative to the pure GaTeCl one, which can be interpreted by the anisotropic structure and by the peculiar electronic structure. Thus, we found that the magnetic ground state, the electronic structure, and the absorption coefficient of M@GaTeCl can be tuned by doping different transition metal M atoms, and the ferroelectricity is still retained, which makes M@GaTeCl a potential multifunctional material in spintronics and optics.