How Much Gluten Does Gluten Free Food Contain?

Gluten free food should contain no gluten right? 
This seems like a simple expectation, but in
reality it is nearly an impossible feat.  This for
sure would limit our food choices.  Currently,
gluten free foods will test to 3ppm at the lowest
and other more reliable tests will detect as low as
5ppm.  Researchers agree that most people with
Celiac disease can safely tolerate up to 20ppm
of gluten.