Z:gnu-www-ja-gpl-2.0-faq--133206-If a program P is released und/en

If a program P is released under the GPL that means any and every part of it can be used under the GPL. If you integrate module Q, and release the combined program P+Q under the GPL, that means any part of P+Q can be used under the GPL. One part of P+Q is Q. So releasing P+Q under the GPL says that Q any part of it can be used under the GPL. Putting it in other words, a user who obtains P+Q under the GPL can delete P, so that just Q remains, still under the GPL.