Global Gag Rule

There is no such qualifier anywhere in the Constitution. It would violate the 1st Amendment for Congress to make any law abridging anyone's freedom of speech, domestically or abroad. In the latter case, of course, there is the further issue of jurisdiction: Congress has no power to make laws for non-citizens outside US territory, so any law which attempted to prohibit speech by non-citizens abroad would be null and void. The same applies to the executive and judicial branches; anyone attempting to enforce such a rule cannot claim any authority to do so based on the US Constitution. Any special status they might claim as an agent of the US government (which would include members of the military as well as diplomats) is conditioned on adhering to the restrictions set down in the Constitution, including the Bill of Rights; Congress authorizing or directing anyone to abridge the freedom of speech—anywhere in the world—would constitute passing an unconstitutional law in violation of the 1st Amendment.

Of course, a person could act *without* any blessing from Congress or the Constitution, but at that point they they're just acting as private individuals outside US jurisdiction on their own initiative and without any special protection, not as representatives of the US government.