Can I substitute Swiss for gruyere in a scalloped potato recipe?

Depending on just what you mean by Swiss, you could have a problem with your cheese separating. A little starch, like cornstarch or potato starch, can help guard against it, as can a little alcohol, like maybe some wine. My mother could make a good fondue with 1960s grocery store Swiss (plus Gruyere), but most people couldn't.

Of course.