Cornflakes on their own are probably ok but there may be other ingredients (eg malt flavourings) that are not gluten free - can you list the ingredients ?
This useful little site gives a listing (on the right hand side of the GF Breakfast page) of all the major brands of breakfast cereal that are gluten free, which might come in useful for you. http://www.the-gluten-free-chef.com/gluten-fre e-breakfast.html