According to the Gluten-Free Living website:
Modified corn starch is an ingredient used in many foods. It is simply corn starch that has been altered by a chemical or enzyme to enable it to perform certain functions in food. For example, corn starch breaks down when heated, but modified corn starch used as a gelling agent better maintains texture in microwaved food. The FDA’s Code of Federal Regulations spells out what can be used to modify corn starch, as well as other starches, and none of the allowed substances contain gluten.
Modified corn starch sometimes appears on a label as plain “modified food starch.” In that case it is still gluten free. The only time you have to worry about modified food starch is when it is made from wheat. On foods regulated by the FDA, wheat has to be specified on the label so the label will say “modified wheat starch,” “modified food starch (wheat),” or wheat will be listed after the ingredients in a “contains” statement.