How to treat pressure in computational chemistry?
Old question, but still seems to attract some interest.
In general, the total pressure of the system is not taken into considerations for corrections. The pressure at which you do the corrections is the partial pressure / concentration of the molecule.
I recommend calculating the thermal correction for $G$ at the temperature of interest and the default pressure ($e.g.$ 1 atm for Gaussian). You can then add the correction for the isothermal expansion/compression to the actual concentration $c_f$ / partial pressure $p_f$:
$$\int_{p_i}^{p_f}\bigg(\frac{\partial G}{\partial p}\bigg)_T dp = RT\ln\bigg(\frac{p_f}{p_i}\bigg) = RT\ln\bigg(\frac{c_f RT}{p_i}\bigg)$$
where $p_i$ is the pressure for the thermal correction ($e.g.$ 1 atm). Note that these formulas are based on the ideal gas approximation, but that's what software like Gaussian does under the hood anyway. Hence, you could in principle directly set the correct pressure and should get the same result, but I find it more convenient to have the same route section for all calculations.
Regarding your multiphasic systems: As you noted, the reaction will most likely take place in the solution, so you have to use concentration of the gas in the solution.