You basically have nothing to lose by pickle staining the wood. I recommend you look up a pre-mixed pickle stain rather than white wash, but the effect is basically similar. Both are a thin stain with thinned white pigment. It will lighten the gray stained wood without fully masking the grain. The final color result is a little unpredictable, and should be adjustible with technique. It should look fine. Pickle staining is often used over dark woods like redwood or cedar to lighten, and the end result is a light to medium color with distinct grain. The pigment is most drawn to the softer wood and the grain is exposed by wiping before it dries. Never tried it on gray, but it should cover. In any event, if it does not work out you can always prime and paint.