We develop a new closed-form representation for the molar Gibbs free energies of gaseous substances. The predicted molar Gibbs free energies are in excellent agreement with the experimental data in a wide temperature range for three gases under consideration, and the average absolute deviations of the predicted results from the experimental data are 0.06155%, 0.0708% and 0.16696% for the gases Cl-2, Br-2, and HCl, respectively. The proposed model is away from the need of a large amount of experimental spectroscopy data, and only related to the dissociation energy, equilibrium bond length, and equilibrium vibration frequency.