The adsorption behavior and electronic transport properties of CO and NH3 molecules on para-C3Si and meta-C3Si monolayers are studied using first-principles calculations and the non-equilibrium Green's function method. The adsorption sites are determined along with their adsorption energies. It turns out that CO and NH3 molecules physisorb on both monolayers. The current-voltage characteristics show that the para-C3Si monolayer can be used to sense CO and NH3 gases with high sensitivity. In contrast to other two-dimensional materials, the sensing mechanism is not based on charge transfer but on the presence of Dirac states and their susceptibility to symmetry-breaking structural distortions.