Biomass gasification in CO2 is a promising thermochemical pathway to assist with growing issues of CO2 in the environment. However, high reaction temperature requirement and the low reaction rate is limiting its development. To resolve these issues, the effect of acid and alkali pretreatment on the pyrolysis and CO2 gasification of pinewood was examined using a semi-batch reactor. The temporal behavior of syngas components, energy, and their yield, and energy efficiency was quantified. Results showed that the decreased alkali and alkaline earth metal (AAEM) content using acid pretreatment was beneficial for the CO and syngas yield, while the effect of the increased AAEM content using alkali pretreatment provided a converse trend. In contrast, CO2-assisted gasification of alkali-pretreated biomass improved the CO and syngas yield due to the catalytic influence of AAEM on the Boudouard reaction, while the acid-washed biomass yielded the lowest syngas yield. During gasification, the syngas yield, energy yield, and overall energy efficiency were enhanced by 83.4 (by wt%), 44.6 (by wt%), and 44.6%, respectively, using alkali pretreatment. The results revealed that alkali pretreatment is an effective catalytic incorporation pathway to improve the syngas, energy output, and reactivity to CO2 gasification.