According to this: http://www.gnu.org/licenses/gpl-3.0.html http://www.gnu.org/licenses/gpl.html (as the striked one will probably link to newer version in the future)
(5. Conveying Modified Source Versions.)
You may convey a work based on the
Program, or the modifications to
produce it from the Program, in the
form of source code under the terms of
section 4, provided that you also meet
all of these conditions:
a) The work must carry prominent notices stating
that you modified it, and giving a
relevant date.
b) The work must carry
prominent notices stating that it is
released under this License and any
conditions added under section 7. This
requirement modifies the requirement
in section 4 to “keep intact all
notices”.
c) You must license the
entire work, as a whole, under this
License to anyone who comes into
possession of a copy. This License
will therefore apply, along with any
applicable section 7 additional terms,
to the whole of the work, and all its
parts, regardless of how they are
packaged. This License gives no
permission to license the work in any
other way, but it does not invalidate
such permission if you have separately
received it.
d) If the work has
interactive user interfaces, each must
display Appropriate Legal Notices;
however, if the Program has
interactive interfaces that do not
display Appropriate Legal Notices,
your work need not make them do so.
A compilation of a covered work with
other separate and independent works,
which are not by their nature
extensions of the covered work, and
which are not combined with it such as
to form a larger program, in or on a
volume of a storage or distribution
medium, is called an “aggregate” if
the compilation and its resulting
copyright are not used to limit the
access or legal rights of the
compilation's users beyond what the
individual works permit. Inclusion of
a covered work in an aggregate does
not cause this License to apply to the
other parts of the aggregate.
Of course this is not legal advice