try http://www.cs.unc.edu/~kupstas/FAQ_recipes.html
First section is gluten-free...
I've made a few gluten free vegan cakes in the last couple of years. One of my DD's best friends is gluten free, the other vegan. Not fun. The gluten free cakes that I've tried from scratch all came out grainy. I think it's something with one of the rice flours. I'm sure there has to be a good scratch gluten free cake out there, but the experimenting was just getting too expensive for me. What I ended up doing was buying a Pamela's cake mix. http://www.pamelasproducts.com/ You can get them at natural food stores, and maybe even at your grocery store. We have them at one of our local grocery stores, but not the others. I just use EnerG egg replacer (it's gluten free and vegan) and vanilla soy milk to complete the recipe.
I wouldn't recommend trying to use an extender that uses sour cream. I had to make a vegan (not gluten free) cake a couple of weeks ago. I tried just doing my regular yellow cake mix doctor cake that I usually do, but with soy milk, egg replacer, & Tofutti "sour cream." It was the most vile thing I have ever tasted. Completely inedible. Perhaps I did something wrong, or there are better vegan sour creams out there, but it was a pretty expensive mistake to make.
Good luck!
I've used Pamela's Chocolate cake mix, and it's good enough for even those who don't have allergies to eat. I used the gluten free booze cake recipe on allrecipes.com and not a crumb was left. The other brand I've found that I really like for allergic people is Namaste. https://www.namastefoods.com/shopping/storefront/cgi-bin/news.cgi?Category=Home
They carry more cake flavors, and kbochick's method would make it vegan, too.
I've also tried scratch gluten free recipes and don't like the results. 
I use the Cherrybrook Kitchen brand mix because it is gluten/egg/dairy/peanut free (all which my grandson are allergic to). I have not been successful trying anything on my own... I purchase these mixes at Whole Foods (the chocolate is good; the white cake is OK).
Remember that you need to wash your equipment REALLY well.. it would help if you used tools that haven't touched gluten before because there can be traces and if the person if really sensitive to it they can have a reaction. This comes from my friend who is very allergic to gluten. So this means your pans, bowls, spatulas, piping bags, tips, cake turner, your serving platter.. EVERYTHING that will come into contact with this cake needs to be cleaned like it has never been cleaned before. The vegan part I wouldn't stress too much about, but the gluten free part is not something to take lightly. The Pamela's mixes get a thumbs up from my friend too. 
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