Ironic,but potatoes are THE best way to "un-salt" a dish so next time (Cause I'm answering you 8 hours too late!) try changing the dish into something else:loose the cheese crust,add some more potatoes,purée them all,add some wheat flour and come up with some protein as filling (ground meet sauté,smoked turkey breast)and put it back in the oven:there you have a version of shepherd's pie and no complains about to much salt.Hope you liked it!

It depends if your potato gratin is already cooked and solidified or still raw before baking?...If still raw, then I would add more potatoes--I have done that to soup and the potatoes seem to work nicely. You will have to add more liquid as well here.

However, if you have pulled your baked gratin out of the oven and ready to serve and it is way oversalty, I would pour in some heated cream/half and half or heavy cream and just stir it and serve it all mixed up. The still hot potatoes will absorb some of the cream, the dish will be a little more soupy (you could toss it back in the oven if you have time), but the cream will take away the extra saltiness.

Hope this helps. I have been in the same pickle and this has worked for me.