I want to calculate the Gibbs free energy of single gas molecule (AsH3) using Gaussian 09, but I am confused between "Thermal correction to Gibbs Free Energy" and "Sum of electronic and thermal Free Energies." which one to use??
You will want to compare “Sum of electronic and thermal Free Energies.". This having been said, I recommend reading carefully the old but always up-to-date white paper from J. W. Ochterski, available here: https://gaussian.com/wp-content/uploads/dl/thermo.pdf .