Gruyere at a pinch, but there really is no substitute for Emmental. The sweet, nutty flavour is almost unique. If the recipe wants Emmental and Gruyere isn't available then don't make the recipe. Or...
Goat cheese, ricotta, curdled cream (Devon Cream), depending on what you are using it for. If you are using it for a salad you can go with crumbled blue or Gorgonzola also.