That is not true!  I quote from the GPL itself:

This is a misunderstanding.

      But
       when you distribute the same sections as part of a whole which is a
       work based on the Program, the distribution of the whole must be on
       the terms of this License, whose permissions for other licensees
       extend to the entire whole, and thus to each and every part
       regardless of who wrote it.

This says that the whole combined work has to be redistributable under
the terms of the GPL, and the permissions of the GPL have to apply to
every part.  So if you *want* to redistribute some part under the GPL,
you are permitted to.

At the same time, if the author of one part wants to give permission
for other use of the part he wrote, he can do so.  Then, if you want,
you can redistribute just that part under the terms he stated.  In
effect, you get a choice of which terms to use.

This actually happens.  There are Lisp files in the Emacs distribution
which say "This file is in the public domain."  You can use those
files like any public domain program.

If you look carefully, you'll see that this is consistent with the
words of the GPL, quoted above.  But I now see that those words are
susceptible to misinterpretation.  I'll think about refining this
wording in the next version of the GPL.

I'm glad you pointed this out to me.