GlutenInBeer--those test-strips you are using might be useful to establish that something is "definitely not safe", but even those that test negative should not be regarded as definitively safe, or even having a high likelihood of being safe.
The 20 PPM standard is based on experiments in cross-contamination, i.e. when whole intact gluten molecules are introduced into the diet at a certain concentration. The studies from which that number is derived suffer from small sample size, but even within the relatively small sample population, 20 PPM is simply the number at which the majority (not the entirety) of the subjects suffered ill effects. There were subjects who suffered severe effects and had to discontinue the experiment at concentrations below 10 PPM, so it's not by any stretch a magic number.
In beer there are a number of variables not found in the production of any other glutenous food. Proteolytic enzymes found naturally in malted barley already show a tendency to degrade barley's hordein molecules into smaller peptide fragments, meaning that some gluten tests will give false negatives even on standard beers (at least, those allowed to undergo a protein rest).
The other problem is that studies on gluten sensitivity focus almost exclusively on wheat; studies on barley (or hordein) are comparatively more rare, and much of what is advised about prolamins are generalized from specific studies on gliadin. Even still, studies are starting to come out demonstrating that there are a variety of different compounds found in glutenous grains that may trigger reactions, and it is not clear if an exhaustive list of these can be made. The upshot of this is that it's not clear if current gluten tests can register the presence of all possible triggering compounds, and in a hydrolyzed food such as wort or beer, these compounds may all be separated from each other, and the absence of some of them does not indicate the absence of all of them.
Simply put, beer is a special kind of food that demands special study, special testing, and special regulation. It is inadvisable to suggest any conclusions about the safety of a beer can be drawn from simply using your test strips, other than that certain beers are "definitely NOT safe".