The present study investigated the co-gasification of two different lignites blended with the forest residue collected from the land restoration activity sites of open-pit mines located in the region of the Ptolemais basin in North Greece performed under the carbon dioxide stream. All samples were devolatilized in a fixed bed unit prior to the gasification evaluations. The gasification evaluations were performed using a thermal analysis system (TG/DTG) operated at temperatures of up to 1000 °C. The reactivity, conversion, cold gas efficiency, and influence of the external catalysts CaO and K₂CO₃ were assessed in the evaluations. The reaction rate of the forest residue was 2–3 folds higher than that of the lignites, with the conversion of the former reaching a value of 96.4% (dry basis), while the conversion of the lignites varied between 43.4% and 51.6%. The peak inflection temperature was in the range of 859–939 °C. The reactivity of the lignite/biomass blends was higher than that of the lignites, and the final conversion was increased by approximately 30%. When individual biochars were impregnated with 30% CaO or K₂CO₃, the process occurred at lower temperatures, and the conversion of the lignites increased by 35%–40% while that of the forest residue reached a value of 100%. The CaO catalyst performed better. Finally, a blend of equal amounts of Kardia lignite or Ahlada lignite and the forest residue with 30% CaO was formulated, which resulted in an 89.6% or 71.7% conversion to carbon monoxide gas, respectively.