What is the most common usage for Gruyère cheese in cooking?

Gruyère cheese is renowned for its excellent melting properties, making it a popular choice for various dishes that require a creamy and smooth texture when melted. It is particularly famous for its role in soups such as French onion soup and in traditional Swiss fondue, where it is melted with white wine and enjoyed with bread or other accompaniments. The cheese's nutty and slightly sweet flavor enhances the overall taste of melted dishes, making it a favored option in these cooking methods.

The other potential usages mentioned do not align with Gruyère's characteristics. While it could theoretically be used in salads, its strong flavor and texture are better suited for hot dishes. Likewise, although Gruyère might occasionally appear in baked goods, it is not commonly a primary ingredient in pastries. Lastly, while it can be used as a topping, especially on gratins, its best-known application remains in melting contexts, aligning perfectly with the choice that highlights its melting qualities.