I would look into a flourless chocolate cake or something that uses an alternative to standard flour, like almond or some other type of nut flour. Unless she's also allergic to nuts!

Check out this one from Whole Foods:

http://www.wholefoodsmarket.com/recipes/753

I agree with Silvrlite. How about a gluten-free linzer torte--The Boston Globe has a recipe mainly with ground nuts and a bit of garbanzo bean flour, which gives nice body. You can use sugar-free jam or fruit spread.

Check out elanaspantry.com.
She has dozens of sugar free gluten free cake and dessert recipes.

Ha! I think you might have the same mother-in-law as I do! Looking forward to the suggestions!

Look into coconut flour for all or the bulk of the cake, whipped cream and fruit for between the layers. Coconut flour generally makes a heavy cake that lends itself to lots of thin layers. Then, on the top layer only, top with cream cheese icing that is made with a little less sugar than usual and top with a coating of raw or toasted coconut chips (larger, flat pieces of coconut). The un-iced sides will be very modern and the whole thing will have only hints of sweetness and not be loaded like a traditional bakery cake. Plus, no gluten and coconut is high in protein and loaded with brain/hormone/fat-fighting saturated fats.

I have made this French-style cake/torte, substituting almond flour for the 1T of flour, and Nigella's clementine cake -- both using Splenda for the sugar. The texture is slightly different for the torte, but the intended gluten-free, diabetic recipients loved both. I'm not a Splenda fan, but thought they were decent.

(Note that the recipient does not have celiac, but a moderate gluten intolerance -- so I'm not sure these are 100% gluten-free in the strictest, celiac sense.)

My mom has Celiac, so I've made quite a few gluten free birthday cakes, and a mixture of 2 parts brown rice flour to 1 part sweet rice flour (like mochi flour, I get a thai brand at the asian market) is a decent substitute for all purpose flour in pretty much any cake recipe.
I have less experience with sugar-free baking, but if she does stevia something like http://www.ehow.com/how_5503652_bake-chocolate-cake-stevia.html or there's one on this site for a cake sweetened with maple syrup http://www.thekitchn.com/thekitchn/dessert/recipe-harvest-cake-with-goat-cheese-frosting-064416
It seems to me that sugar is more structurally important to a cake than gluten is, so find a sugar free recipe and just substitute the flour.

I second the coconut flour suggestion! I'm allergic to wheat and coconut flour is the best..... And if you want to make the cake a little lighter, you can do a sort of souffle type thing where you whip the egg whites and it makes the cake a lot lighter.

I've had great success with the original Babycakes cookbook, it's also vegan but still tasty! http://www.babycakesnyc.com/books.html

I've made cheesecake with maple syrup before, and it's quite lovely.

In terms of gluten-free (though not sugar-free), this almond cake is glorious: http://www.sevenspoons.net/blog/2010/3/3/snow-globe.html

What about a pavlova? I'm sure you could use an alternative sweetener for the meringue. The fruit and whipped cream don't need to be sweetened.

I've been baking gluten-free for a few years now. Carrot cake can be made gluten-free with very good results and would work well using applesauce for some of the sweetness (just reduce oil and liquid a little), and the rest of the sugar could come from pureed dates/other dried fruit and/or maple. One of the advantages of carrot cake is that it doesn't need to be especially sweet to taste good. I believe the recipe I've used for reference was from the Joy of Cooking.

For icing, you could make a nice glaze with a bit of milk and/or cream cheese mixed with agave, maple, or honey and pour it over while the cake is still a bit warm.

One last thought: since she is ok with maple & agave, what about raw sugar? There's not a huge difference, nutritionally, between raw sugar and other "natural"/unrefined sweeteners, and using raw sugar for some of the sweetener would probably help give the cake or glaze a more familiar texture.

I make gluten free birthday cakes for my daughter (http://whatsheate.com/post/1663045841/11-23-10) and I have used Bob's Red Mill gluten-free flour on occasion. It works like a charm in a pinch, and is great when you don't have time to make your own gluten-free mix.

For the sweetener use stevia, which I have been using in baked goods for years. The trick with stevia is to make a really flavorful icing. Perhaps make a raspberry cake topped with lemon icing and fresh raspberries on top.

Good luck with the cake, please let us know how it turns out!!!

Although I haven't made it yet, I have been eyeing this Raw Cashew Dreamcake (vegan, raw, gluten free "cheesecake"). The sweetness comes from medjool dates and honey, and the cheesiness comes from ground cashews and coconut fat. It looks absolutely droolworthy and I'm excited to try it!

http://mynewroots.blogspot.com/2011/01/raw-cashew-dreamcake.html

I've made gorgeous carrot cake with gf flours - find one that uses pineapple in the recipe as that's going to keep it moist. And just hit the google - seriously, gluten free is NOT hard to find, and ones that use agave, honey, or other non-white sugar is also SO not hard to find. Not a tall order at all actually.

At what point does it cease to become cake? I wonder this often when cooking for my inlaws, who are entirely fat free and vegetarian. Sometimes, a nice piece of fruit sounds better than whatever fake substituted faux-whatever that they request. I feel for you!

I've made gluten free tortes using erythritol (natural and 0 cal) and they turned out great! I'm sure xylitol would work as well! Agave happens to be very high in fructose so i wouldn't recommend it. You can try coconut sugar though. it's lower GI and is really delicious :-)

Ther are all kinds of sugar substitutes you could use (agave, date sugar, maple syrup, etc) and there are a ton of great gluten free cake recipes easily available online (I should know, I cook almost entirely from scratch and am gluten free) but if you're nervous about working with unusual ingredients that may be costly or hard to find you can always jazz up a simple box of Betty Crocker gluten free cake mix. It's not my favorite cake mix but it's good in a pinch and fairly easy to find. Check it out at http://www.bettycrocker.com/products/gluten-free-baking-mixes. If you need help with a recipe or have questions about gluten free coking you're welcome to stop by my site blessedblogger.com and should me an email.

I love the above comment recommending raw desserts! (As in raw "cheesecake," etc.) But if you want to go a more traditional route:

I used to use agave in baking quite a bit; as a general rule, use about 1/4 less agave than you would sugar (ie, in a recipe calling for 1 C sugar, use 3/4 C agave), and decrease the liquid in the recipe by 1/3. (ie, for 1 C milk, use 2/3 C milk.)

Almost all grocery stores now have a basic gluten free flour mix. I recommend buying some; that way you can use "normal" recipes and just adjust.

http://www.simplysugarandglutenfree.com/