Sugar Free / gluten free options available?-
I wouldn't say there are a lot of choices available, but there are some. The cruise lines offer "healthy choices" entrees. These very from cruiseline to cruiseline but most will be identified on the menu as such. They also serve vegatarian entrees. As far as sugar free goes, I have seen deserts that are sugar free being offered (made with sugar substitutes) on Carnival menus and I imagine many of the cruise lines are offering similar items.
If you have special dietary needs, you should discuss them with the head waiter the first day of the cruise so that they can prepare them if possible. You can read Carnival's note about special diet needs here.
Carnival Cruise Lines:
Fantasy 90, Celebration 95, Victory 05, Conquest 05, Spirit 6/06, Freedom 6/08, Holiday 7/08, Victory 6/09, Valor 9/09, Dream 6/10, Splendor 8/10,Conquest 8/11,Magic 11/11,Triumph 4/12,Spirit 7/12,Freedom 11/12, Triumph 2/13 - The fire cruise
NCL Dream October 2006, RCCL Mariner OTS June 2007, Celebrity Silhouette 2013