Tetrahedrally bonded amorphous carbon (ta-C) thin films were prepared by mass selected ion beam deposition (MSIBD) using 100 eV carbon ions at room temperature. Gadolinium, a magnetic rare earth element, was implanted as a dopant into ta-C with two different fluences. The doping level of the ta-C:Gdx layers was in maximum 4 or 7 at.%, respectively. The Gd is believed to be electrically activated in as-implanted films, although contributions from the sp2 sites cannot be ruled out. A transition temperature (T') was found, below which there is a large negative magnetoresistance (MR) with a strong temperature dependence. Thermal annealing greatly increases the sample conductivity due to the increase of sp2 sites. However, the MR persists at least up to an annealing temperature of 500°C. Magnetically, the Gd dopants behave like non-interacting local moments. Similarities and differences in physical properties between the ta-C:Gdx films and other Gd doped amorphous semiconductors are compared.