IF you want to research a food ingredient, you should visit the Food & Drug Administration at
http://www.fda.gov/. They are currently working on a proposed rule which states that any food with an ingredient that “results in the presence of 20 parts per million (ppm) or more gluten in the food; or 20 ppm or more gluten” cannot be labeled as gluten free.
Diet Coke contains caramel coloring but different people have different reactions to different foods. Some people believe that many of the Coke products are safe to drink. Others think you should stay away from any product containing caramel coloring. Ultimately, it's going to be up to you to test out for yourself.
Look at the Gluten Free Club, keep asking the questions and start to do your own research. You need to educate yourself on this condition more than anything...trial and error will be your best guide.