Is there any reason why this cannot be made a few hrs ahead and served room temp? I ask b/c I saw the recipe says serve immediately. Thanks!

If you like other cold gratins, go ahead and enjoy.

For me, the main point of all gratins is the fact that they are close to bubbling hot and above all the top is crispy. If you leave it aside and serve at room temperature, the top will start to absorb the moisture from the gratin and soften.

Entirely your call based on what you expect from a gratin and if you care for the crispy & crunchy top.