Graphene based gaseous sensor has been shown experimentally to have high potential for detecting gas molecules by measuring the conductance change as induced by the adsorbed molecules on graphene. Currently, there is no atomistic based theoretical model that is able to adequately explain the working of the sensor. Based on a single-atom statistical model, the kinetics of adsorption and desorption of gas molecules on graphene was investigated in the present work, and an atomistic mechanism for the functioning of the sensor is proposed which relies on the change of the conductance when the adsorbed molecules diffuse into the defective sites. This mechanism is in agreement with previous experimental measurements and indicates some possible ways to increase the sensitivity and response speed of the sensor.