I don't use PS, but the concept is the same. Try duplicating your base. decreasing saturation by about 80 percent and increasing lightness by about 70. Add a slightly visible noise layer, then merge the noise layer with your main duplicate. Use your gradient mask layer to create the coverage you desire. As an added step, dodge or burn your mask to give the final appearance of fog's randomness.
These aren't great, but they show the results of the above.
Before
After