Not OP, but I think I get what they're trying to say.
The GPL requires others to follow its terms, because that's the only license of the code that's available to them. There is no clause of the GPL or any software license that I'm aware of that prevents you from doing whatever you want with your own code under different terms.
Therefore, you can do something negative (sell software that uses changes to GPL'd code without providing those sources to others when they ask for it), while maintaining a positive image ("look, I license my code under the GPL because I care about software freedom").
You can see this happening in practice with software that has a "community" edition available under the GPL, but a paid version that's not GPL'd. The holder of the copyright gets to profit off of other people's well meaning contributions (which almost certainly will have had their copyright assigned to the original copyright holder), but the people who contributed don't get the benefit of the copyright holder's changes to that code that are only available to paid customers.
You can argue that those contributors made their contributions knowing that would happen, but the fact that it is possible somewhat undermines the entire point of the free software movement.
Do you have an example of what you mean by "gag", in context of copyright?
Do you have an example of how GPL or other licenses allows you to do something negative while maintaining a positive image?