The accelerated research and scale-up of microalgal processes especially those involving genetically modified (GM) strains for producing high-value specialty chemicals will generate biomass residues with limited disposal routes. To extract value from these GM waste streams, thermochemical conversion can transform them into useful energy products. In particular, gasification is a promising destructive pathway for the generation of biosyngas. However, several aspects remain underexplored, including the effect of introducing steam and CO2 in the gasifying agent on the evolution and characteristics of syngas and tar. This study explored the gasification of dried GM Cyanidioschyzon merolae biomass under mixed air/steam/CO2 atmospheres in a fixed-bed tubular reactor at 850−1000 °C. In addition to analyzing the syngas yields and composition, the resulting tar was comprehensively characterized by GC−MS and FT-ICR-MS to elucidate its molecular structure. It was found that steam injection significantly enhanced syngas quality, increasing the H2/CO ratio up to 1.5 at 1000 °C; however, it also led to higher tar yields. In contrast, introducing CO2 in the gasifying medium reduced syngas yields. Across all conditions, tar samples predominantly featured condensed aromatic structures (350−450 Da, DBE/C > 0.7). Higher temperatures strongly influenced tar chemistry by promoting the formation of single nitrogen-containing aromatics and pure aromatic hydrocarbons. However, across the gasifying atmospheres tested, tar composition remained similar. These findings offer molecular-level insights into how steam and CO2 influence product distribution and tar chemistry during microalgae gasification, informing future strategies for tar mitigation and process optimization.