Utilization of sewage sludge, in a way friendly to the environment, is still a subject of intensive investigation. One of the possible solutions is the gasification of sewage sludge. Torrefaction and the use of additive can enhance this process. This study presents the results of the gasification of raw and mildly torrefied sewage sludge, as well as mildly torrefied sewage sludge with the addition of calcium carbonate. Overall, the torrefaction in mild conditions had little influence on the composition of the obtained gas. The influence was much more visible when 5% of calcium carbonate was added. The additive caused the increased average concentration of CH4 and decreased average concentration of H2 in the producer gas. The use of additive also improved the kinetics of the gasification of torrefied sewage sludge. Moreover, the tar deposition diagram confirmed that the use of calcium carbonate enhanced the quality of the produced gas by decreasing the content of the compounds, with a melting point higher than 30 °C, thus minimizing the deposition problem.