The activity coefficients of infinitely diluted solutes in a solvent can be calculated with Properties → Activity coefficients.
Select Properties → Activity coefficients Select 'water' for the first component in Solvent Press 'Run'
If one does not supply a density of the solvent in the input the program calculates the density of the solvent by dividing the mass of a molecule with its COSMO volume. Note that the calculated activity coefficients do not depend on this density. One may improve the results for the calculation of the Henry constants, if one includes pure compound vapor pressures in the 'Compounds' window (Compounds → Compounds).
The result of the calculation is given in the form of a table.
Relevant for the calculation of ΔG from the gas phase to the solvated phase is the reference state, used here is 1 mol/L both in both phases.