The main purpose of the present work was to evaluate the efficiency of the gasification process of three different types of agro-forest biomass residue (rapeseed, softwood, and sunflower husks) along with the characterization of size-segregated particulates’ emissions. The experiments were carried out in a drop tube furnace (DTF), using two different gasifying agents (O2/N2 and O2/N2/CO2) at atmospheric pressure and a constant temperature of 1000 °C. In focus was the effect of biomass and the gasifying agent on syngas composition (CO, H2, CH4, and CO2), cold gas and carbon conversion efficiency, and on the emissions of by-products, such as particulate matter (PM), known for having negative environmental and health impacts. The collected particulates were characterized by SEM/EDS and XPS analysis. The results reveal that: (i) the introduction of CO2 increased the production of CO and CH4 and syngas’ lower heating value (LHV), thus leading to higher cold gas and carbon conversion efficiency; (ii) CO2 decreased the production of H2, leading to lower H2/CO ratio (between 0.25 and 0.9). Therefore, the generated syngas is suitable for the synthesis of higher hydrocarbons, (iii) CO2 lowered the emissions of char (cyclone) particles but increased the overall PM10−0.3. Submicron size PM was the dominant fraction (PM1−0.3) in O2/N2 and (PM1.6−0.3) in O2/N2/CO2. Unimodal PM size distribution was observed, except for sunflower husks gasification in O2/N2/CO2; (iv) the SEM/EDS and XPS analysis confirmed that submicron-sized PM1−0.3 contain above 80% of carbon associated to soot, due to incomplete oxidation, whereas in cyclone (char) particles, carbon decreased to about 50%. The SEM/EDS results showed that K and Cl are typical constituents of the submicron size PM, whereas the alkaline earth metals were detected mainly in fine and coarse particulates. Detailed analysis of the XPS (C1s) spectra showed that the most common oxygen-containing groups on the PM1 surface were carbonyl and carboxyl.