The Betty Crocker GF mixes make 12 cupcakes. Most frosting recipes should be GF, except the cooked flour ones obviously. If you are only do this one order GF I'd just buy the Betty Crocker mix and doctor it up (The Cake Doctor Does GF has a bunch of recipes). If you plan to do a lot of GF baking then you will want to find a flour blend that works for you. The mixes are expensive, but buying a bunch of GF flours to blend together and an entire bag of xanthan gum for just 1 cake is going to cost a whole lot more. The xanthan gum alone is about $10 and you only need a tsp or so.
Just make sure to be extremely careful about cross-contamination, including waiting 24 hours after flour has been in the baking area.
My husband eats GF, and I just recently made him a cake. I used the Betty Crocker yellow cake mix and doctored it up using the durable 3D cake recipe on here. It turned out pretty good 
I have celiac disease and only bake gluten free. My first choice is to bake from scratch, but if I doctor up a Betty Crocker mix it tastes good too. I don't care for them plain.
I use this recipe from the Cake Doctor Does GF and I think it's good. I actually used it this weekend for a birthday cake for my niece and no one is GF except me and they all loved it.
1 Betty Crocker yellow cake mix
1/2 a vanilla instant pudding mix
3/4 cup milk
1 stick unsalted butter
3 large eggs
1 tbsp vanilla
Yes, I use it with the Betty Crocker GF yellow cake mix
Quote by @%username% on %date%
%body%