Wheat, rye, barley
Wheat rye and barley contain gluten
The most recent consensus is that most celiac patients can eat oats; the problems some had with oats were due to contamination by other grains. Be careful the oats are pure if you try to eat them. All other foods do not contain gluten, so genetically modified foods and non-modified foods could be gluten free, as long as they aren't the grains above.