What can I substitute for Gruyère cheese?-- D
To some extent, that depends on what you are using it for. In fondue, Emmental would be the standard substitution. If you can't find Emmental, look for generic Swiss Cheese. The Food Substitutions Bible suggests you can also use Comté or Beaufort as a general substitute for Gruyère.
For melting on top of one of my all time favorite potato dishes, Potato Flan with Caramelized Onions, I really like Jarlsberg, since its slightly stronger, nutty flavor stands up well to the complexity of the dish. The original recipe called for Gruyère. For grilling or melting onto potatoes or vegetables, you could also use Raclette from France or Switzerland.
And for eating out of hand, try any of the above as well as Maasdam or Edam.
If you live somewhere that has a good, knowledgeable cheese store, like the Bulk Cheese Warehouse in Saskatoon, go in and tell them what you are making. They may have many more interesting suggestions. If you are looking for a cheese to go with a particular food, say maybe to serve alongside a Peach Cobbler, tell them. They may be able to suggest an interesting pairing. If you aren't sure of a recommendation, most cheesemongers will be happy to give you a taste of any cheese they suggest.