For soups, an old trick to absorb some of the salt is to put sliced raw potatoes into the mixture (which will soak up some of the salt), and then remove the potato slices and taste.  For already prepared Au Gratin potatoes, unfortunately, you might just be out of luck.

The answer to this question is pretty dependent upon knowing how the potatoes were prepared.  Normally, I’d suggest slicing the potatoes thin on a mandolin or commercial meat slicer, immediately rinsing in a plastic bucket until the excess starches and sugars leave the surfaces (the water will be clear instead of cloudy), and then briefly storing in chilled water before draining or spin drying (using a salad spinner to rid the excess water).  So, no salt added at this stage.  Once cream or grated cheese is introduced to the dish, the salt is being added to the recipe.  Be sure to taste the cheese to make sure it’s not too salty.  If it is, consider adding another blend that is not so salty (such as a mild melting cheese like mozzarella or Monterey Jack) to the parmesan to cut out some of the salt from the aged parmesan, or mix the cream with a low-fat or 2% milk to dilute the salt.