Because of environmental and safety concerns, there is a large need of gas sensing devices. Semiconductor metal oxides, like tin dioxide, are widely used for such applications. In these devices, the gas detection is usually performed by measuring conductivity variations of the oxide sensing layer. Indeed, gas adsorption and surface chemical reactions result in formal electron transfers, modifying the number of charge carriers. In this work, we have performed DFT calculations to characterize different chemical reactions involved in the detection of CO and CO2 by SnO2: CO and CO2 adsorption, SnO2 reduction by CO and its re-oxidation by O2, and the formation of carbonates in the case of CO2. In each case, we have calculated the reaction energy and its activation barrier but also the resulting charge transfer in order to evaluate the sensor electrical response. We find that CO detection is easy thanks to low barrier exothermic reactions and large associated charge transfers. In particular, SnO2 reduction results in the formation of surface oxygen vacancies that formally behave as two electrons in the conduction band. On the other hand, CO2 detection is troublesome due to weakly exothermic reactions and ten times smaller charge transfers. Finally, we study the influence of H2O on the reaction mechanisms and the charge transfers in order to evaluate its interfering capabilities.