If your idea of gluten free cake is a grainy, crumbly cake, you haven't tried some of the new mixes. I've had a couple that were wonderful cakes--light and moist and full of flavor.
Unfortunately, the one I got for my husband's birthday was not one of them. I tried a chocolate cake mix, made by Cherrybrook Kitchen; it has the advantage of being vegan. It also had the advantage of being the only one in the particular store I went to. That's all I can really say about it. The texture was very fine, and it was light, and it was moist, but these did not overcome the grainy texture of the cake itself. I was very disappointed.
The good thing about this particular cake is that most of it is sitting on the kitchen counter. Neither of us like it well enough to stick it in the freezer for later, which means we will not eat it. That's better for our health and our diet, but not very good for a birthday!