Suing a patent troll for GPL violations is impossible by definition. A patent troll is a non-practicing entity, so they don't produce anything (but lawsuits) and thus they don't distribute software and conversely can't be sued for GPL violations.

If whatever company that is suing on patent grounds is also producing software (like this current case), then they are not a patent troll by the most common definition of the term but just plain aholes.