Quoting from the nettle manual:
Nettle is distributed under the GNU General Public License (GPL) (see the
file COPYING for details). However, most of the individual files are dual
licensed under less restrictive licenses like the GNU Lesser General Public
License (LGPL), or are in the public domain. This means that if you don't
use the parts of nettle that are GPL-only, you have the option to use the
Nettle library just as if it were licensed under the LGPL. To find the
current status of particular files, you have to read the copyright notices
at the top of the files.