Jules Gluten Free cake is considered to be very good! Yes, it means buying her flour (which is expensive), but its worth it as she's figured out the right proportions to make it work well.
http://blog.julesglutenfree.com/2010/11/whitebirthday-cake/
Other recipe I've found doesn't use flour at all: (no opinions, as I've not tried it)
http://glutenfreegoddess.blogspot.com/2007/01/flourless-chocolate-cake_11.html
I hope this helps you. My sister has used Bob's Red Mill Brownie Mix and that was good. So, their cake mix should work as well.
The problem with GF baking, of course, is the missing gluten which adds moisture and acts as a binder when combined with egg. Commercial products are the "tried and tested" mixes that have been found to work the best. I've been Gluten free for several years, but is still learning GF baking.