The GPL does not require public distribution of derivative works; only that the original licensor could not prevent licensees (even indirect ones) from performing redistribution. This is used all the time: within corporations it's normal to patch/fix GPL'd software and not distribute the changes outside of the organization, with the caveat that distribution could not be stopped by GPL licensing terms; usually, instead, it will be stopped by employment contracts.

The GPL has gone through a lot of hoops to warp itself in such a way that it takes a normally restriction-based system (licensing) and turns it into a limited guarantee of freedom from restrictions. But, it can't really subvert something like a gag order.

Consider the case of a dual license, where one license is a GPL and another license is some permissive noncommercial license. The restrictions of both licenses apply, and thus, the result is a composite set of licensing terms that resemble a GPL with commercial use restrictions. The GPL's interaction with any other set of laws that restrict distribution will operate the same way.