If I am making a gluten free cake... I am planning on making standard buttercream icing... Butter and icing sugar/powdered sugar ... is this gluten free???
Powdered sugar is usually gluten-free, but it is important to always check the label. Some brands even label the package as gluten-free. Most commercial powdered sugars are blended with corn starch to keep them fluffy. Occasionally, some manufacturers have been known to use wheat products instead of corn starch, so ALWAYS check the label every time you buy!
Read more: http://wiki.answers.com/Q/Is_powdered_sugar_gluten_free#ixzz1MCfEoXeB
The best thing to do to be sure is to use a top brand like Dominos or C&H. If in doubt call the 800 number on the package and ask them.
To OP - Most powdered sugar is gluten free, just check the ingredients to be sure. If your customer is also allergic to corn products, try to find sugar that uses tapioca starch instead of cornstarch - several organic brands do this.
Aviles2005, all of the gluten-free cake recipes I have used are also dairy and egg free so they may not be what you are looking for. If you are getting different results with the same mixes and recipes, maybe you are mixing at different speeds or for different amounts of time? You do not want to over-mix gluten free batter or it gets too dense. Even though I have a Kitchen Aid mixer I often use a spoon or a hand mixer for gluten free batters and just mix until it is uniform with no lumps. I hope this helps.