The first vector of diffusion of the CGAL sources is the publication of the
source tarballs of CGAL releases, twice a year. The file
Installation/LICENSE, in the Git repository, is the file LICENSE at the
root of the source tarballs. It describes the license of all the files of
the source tarballs.
A lot of files in the Git repository are not distributed in the source
tarballs, for examples all the files in the doc/ and test/
sub-directories of CGAL packages. For all those files, unless they have an
explicit license notice, the license is the GNU General Public License (as
published by the Free Software Foundation; either version 3 of the License
or (at your option) any later version). The text of that license can be
found in the file Installation/LICENSE.GPL.