The GPL sets requirements for obtaining authorization to copy and
distribute the covered work. Such authorization may be finely
divided, as when an author sells paperback and hardcover rights
to separate publishers. The GPL does not permit code covered by
it to be copied and distributed as part of a combined work unless
that work as a whole is distributed under the GPL. The exclusive
right that the GPL authorizes is only on how the covered work
itself may be copied and distributed when it is part of a
combined work.