The GPL in particular travels with a buzzing cloud of misinformation and disinformation spread by opponents and fanbois alike. If these questions are on-topic, you can bet that the numerical majority of answers will come from these enthusiastic sources. They don't in fact help the askers or the site. This is my prime argument for 'off-topic'. Common-sense answers are nearly guaranteed to be washed away in the tide of fudd.
The answer, 'ask a lawyer,' is often short-hand for 'The question you ask is, in fact, legally complex. It doesn't have a quick answer.'
If someone asked for a linear-time algorithm for the travelling salesman problem, would not 'there isn't one' be a valid answer? If someone asks for an informal answer to a question about the GPL, I submit that it's the same thing.
Or, via another approach, I've taken to giving answers that consist of a suggestion to read Larry Rosen's book on the subject. He is a very qualified lawyer, and his whole book is available, for free, on the interweb.
As for the original question of 'GPL in closed source,' of course I'd ask a lawyer. There are many different software usage and distribution schemes that come under the completely uninformative heading of 'closed source.' Some, even Stallman would approve. Others clearly violate the terms of the GPL, but many are somewhere in the mushy middle.
Furthermore, license violations are not a capital offense. They are not even a criminal offense. They are a civil matter. That means (a) that any court of clowns in Texas can decide anything, and (b) businesses and individuals have to make decisions about acceptable risk. If you have a lot of money on the line, you would be crazy to look here for an answer. If you have nothing to lose, well, I suppose the answers here might be a little better then haruspices.