Personally, the GPL comes as some difficult reading concerning subjects like this. Let me point out why...
For example, if you distribute copies of such a program, whether gratis or for a fee, you must give the recipients all the rights that you have. You must make sure that they, too, receive or can get the source code. And you must show them these terms so they know their rights.
3. You may copy and distribute the Program (or a work based on it, under Section 2) in object code or executable form under the terms of Sections 1 and 2 above provided that you also do one of the following:
a) Accompany it with the complete corresponding machine-readable source code, which must be distributed under the terms of Sections 1 and 2 above on a medium customarily used for software interchange; or,
b) Accompany it with a written offer, valid for at least three years, to give any third party, for a charge no more than your cost of physically performing source distribution, a complete machine-readable copy of the corresponding source code, to be distributed under the terms of Sections 1 and 2 above on a medium customarily used for software interchange; or,
c) Accompany it with the information you received as to the offer to distribute corresponding source code. (This alternative is allowed only for noncommercial distribution and only if you received the program in object code or executable form with such an offer, in accord with Subsection b above.)
Contrast the bold of the former quote with the three sub-parts (a, b, c) of the latter. Doesn't this seem to be ambiguous? It seems akin to saying: "either you must distribute the source code yourself,
or you may use any other third-party to do it for you".
Obviously the easiest way to make sure one is complying with the license is to make sure you distribute the source personally, on the same server(s) as the binary. However, the lack of clarity generates some unease about whether that
must be done. Reasonably speaking, what purpose is there to distributing the source code to someone else's project on your own bandwidth? Of course, you could say the same about binaries -- but they are in far greater demand.