Well, I can only tell you about my experiences with a client that asked me to make gluten free cakes for her. First off, what kind of tolerance does your customer have? If they react to even the slightest traces of gluten, you can contaminate the product if you are not working in a dedicated kitchen. My client had somewhat tolerance so I had to make hers 24 hours after baking my regular items and made sure that even the oven and cabinet doors were wiped down. Even the cleanest pans/mixer/oven can harbor small amounts if used in regular baking. What made mine particularly challenging is that she was also allergic to cornstarch!
Now, having said that, there is a product called Pamela's that a lot of people like. You can add flavors to it (and they have that on the package) to make it your own. My client did not care for the mix, so I had to make mine from scratch. I made an orange cake w/almond flour and a lemon cake w/a lemon glaze for her. I will be more than happy to pm them to you if you are interested.
King Arthur also has great products but it has cornstarch in a lot of their products. Now, keep in mind that gluten free products have a different texture and it can get expensive! I lost money in the end because she is now too far away to order from me and I had to buy all of the products to make two cakes! I don't think that I have the picture of them on here but I have them on my FB page "Rocky Hill Cakes".
I would also ask Jason_Kraft on here. He always has good advice on making cakes for people with allergies.