Our opinion after numerous cruises with NCL is that Le Bistro and Ocean Blu are the best of what we've tried. Cagney's has gone downhill, although it really varies ship to ship and depending on your server.
That being said, La Cucina can also be very good. Again, really varies ship to ship.
Gluten free is not an issue if you inform maître de on day one (or others may have details on how to do it ahead of time). They will have gluten free bread and pasta options for you, and it will be made in a separate kitchen so that there is no cross contamination. My self and a friend have both done gluten free successfully on a number of cruises and my celiac sister-in-law has done it on two cruises. The best way to get what you want is to pre-order the night before for the next day's dinner. That way they have time to accommodate your requests. Once they made a special gluten free lemon cheesecake for my sister-in-law and me.