Nothing to add to the explanations in the answers already given, but here are instructions for how to shape your source file headers (source):
2.2 Adding GPL’d modifications to permissive-licensed files
A more complicated case occurs when a developer makes copyrightable changes to a permissive-licensed file that the developer is incorporating into a GPL’d program. Developers in this situation typically apply the GPL to their modifications. (However, it is possible for the developer instead to contribute new code under permissive terms, such as the permissive license that governs the unmodified file. We discuss that case in § 2.3.)
Even though the permissive license of the external project grants legal permission to incorporate code from that project into a GPL’d project, the developer of the GPL’d project must nonetheless comply with the notice preservation requirement in the permissive license. In a project that uses the file-by-file method, a developer who makes copyrightable modifications to a permissive-licensed file should place a new copyright notice and permission notice above the existing one and should make clear that the developer has modified the file. The top of the file will then appear as follows:
* Copyright (c) 2007 GPL Project Developer Who Made Changes
* This file is free software: you may copy, redistribute and/or modify it
* under the terms of the GNU General Public License as published by the
* Free Software Foundation, either version 2 of the License, or (at your
* option) any later version.
* This file is distributed in the hope that it will be useful, but
* WITHOUT ANY WARRANTY; without even the implied warranty of
* MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
* General Public License for more details.
* You should have received a copy of the GNU General Public License
* along with this program. If not, see .
* This file incorporates work covered by the following copyright and
* permission notice:
* Copyright (c) YEARS_LIST, Permissive Contributor1
* Copyright (c) YEARS_LIST, Permissive Contributor2
* Permission to use, copy, modify, and/or distribute this software
* for any purpose with or without fee is hereby granted, provided
* that the above copyright notice and this permission notice appear
* in all copies.
* THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL
* WARRANTIES WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED
* WARRANTIES OF MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE
* AUTHOR BE LIABLE FOR ANY SPECIAL, DIRECT, INDIRECT, OR
* CONSEQUENTIAL DAMAGES OR ANY DAMAGES WHATSOEVER RESULTING FROM LOSS
* OF USE, DATA OR PROFITS, WHETHER IN AN ACTION OF CONTRACT,
* NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF OR IN
* CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
It is very important that the developer preserve the entire copyright notice, permission notice, and warranty disclaimer as they appeared in the original code, as required by the permissive license. We sometimes see GPL notices mixed in with permissive license notices—a confusing practice that obscures both the provenance of the code and the precise permissions that were granted by the various copyright holders listed in the notices. When different copyright holders have released their contributions under different terms, the terms that each has placed on his particular contribution should be specified. We recommend making a clear separation and using indentation, as in the example above.
This manner of organizing the notices in the file makes it convenient for developers to choose whether to contribute under permissive terms or under the GPL. If they wish to make their contributions available under permissive terms, they can add their copyright notices to the lower group. If they wish to contribute under the GPL, they can add their copyright notices at the top. Note, however, that in a single source file it is typically very difficult, and often completely infeasible, to determine which parts of such a file are covered by permissive terms. If the goal is to make additional code available under permissive terms only, the method described in § 2.3 should be used.