This study investigates the effect of shielding gas composition on both hardness and microstructure of high-strength low-alloy steel (HSLA) walls made using Wire Arc Additive Manufacturing (WAAM) with Gas Metal Arc Welding (GMAW) in Cold Metal Transfer (CMT) mode. In this study, two different shielding gas compositions with different CO2 concentrations were used. Shielding gases in this study were 18% CO2 + 82% Ar, and 8% CO2 + 92% Ar. The aim is to evaluate the impact on the material’s microstructure and hardness. Results showed that no significant changes in microstructure and hardness were found between walls produced using the two shielding gases. The wall produced using 8% CO2, however, showed slightly higher hardness than that produced using 18% CO2. The optical microscope showed ferritic microstructure for both walls with a bit higher bainite in the case of the wall made using 8% CO2. Different oxidation extent of alloying elements in the walls due to the use of different content of CO2 in the shielding gas may be responsible for slightly different hardness, and microstructures observed.