Could a person eat enough processed gluten-free goods with 20 ppm of gluten (or 10 ppm or less in the case of GFCO certified products) to create a build-up of gluten in the body that would be problematic?

If a person eats products that contain gluten at some ppm level greater than zero, then the more of those products consumed, the more gluten would be ingestedThe amount of gluten ingested via products labeled gluten-free in accordance with FDA regulations, or that are GFCO third-party certified as gluten-free, would likely not be problematic given the normal amount of food consumed by a person in a day. Here’s why: 

Research has indicated that 10 mg per day of gluten is a safe level of consumption for individuals with celiac disease. Read the study here.

Using 10mg per day as a safe threshold, a person would have to eat half a kilogram (1.1 lbs.) of products containing 20 ppm of gluten to reach that 10 mg mark during the course of a day. If they were consuming a GFCO-certified product containing 10 ppm of gluten, that food volume goes up to a full kilogram (2.2 lbs.) to get to 10 mg of gluten 

Keep in mind that most gluten-free products don’t have gluten levels anywhere near either threshold. It would take quite a bit of eating to exceed 10 mg of gluten per day and cause a problem.