The main purpose of the present article is an investigation of electronic and magnetic properties of g-SiC2, decorated by 3d transition metals (TM), for two different concentrations of V, Mn, Co or Ni atoms. To pursue this aim, we employ the spin-polarized density functional theory (DFT), in which the spin degree of freedom is explicitly taken into account. The results of our first-principle calculations demonstrate the stable geometrical configurations, the corresponding geometrical parameters and the adsorption and diffusion energies of each combination. The adsorption of TM onto g-SiC2 is then shown to be of chemisorption type, causing, among others, an out-of-plane corrugation. We proceed by calculating the charge transfer (total and spin-distinct) and charge redistributions, under the effect of adsorption, for the aforementioned combinations. Our results indicate that in all of the considered combinations, the adsorbate TM atoms lose electrons to the g-SiC2 sheet. We, moreover, calculate the spin-distinct electronic band structures (SDEBS), as well as densities of states (SDDOS) from which we conclude that for a low (large) concentration of TM adatoms, the combination is a ferromagnetic half-metal (FMHM) for V (for both concentration), it is a FMHM (ferromagnetic metal (FMM)) for Mn. A more important result of our calculated SDEBS and SDDOS is that when Co is adsorbed onto g-SiC2 the combination behaves as a ferromagnetic bipolar material at low concentration of the adsorbate, while for larger concentrations it becomes an antiferromagnetic semiconductor. In contrast, we show that the combination of Ni plus g-SiC2 is of nonmagnetic semiconductor nature, for both concentrations. Our calculations then demonstrate a novel path to design materials with adjustable magnetic properties, ranging from half-metallicity, bipolarity to nonmagnetic, with potential use in spintronic applications.