Even though the individual source files in this repository are under
GNU General Public License version 2 or later, the final binaries were
being implicitly upgraded to version 3 or later of the license by
virtue of linking against libraries like Babl and GEGL that are under
GNU Lesser General Public License version 3 or later. Since GPLv2 does
not permit combinations with LGPLv3, the only way to distribute the
combined work was under GPLv3+.

See: https://www.gnu.org/licenses/gpl-faq.html#AllCompatibility

Let's clarify this by explicitly stating that this project is under
GPLv3+ instead of pushing the burden of understanding how the code is
linked together, and the licenses involved, on individual distributors.