The page OP links to has at the top "LICENSES". If you click on it you go here: https://www.gnu.org/licenses/licenses.html

On that page it says: "We normally use the GNU General Public License (GNU GPL), specifying version 3 or any later version, but occasionally we use other free software licenses."

If you click on "GNU General Public License (GNU GPL)" you go to https://www.gnu.org/licenses/licenses.html#GPL which shows GPL v3. And there is a link to older versions here: https://www.gnu.org/licenses/old-licenses/old-licenses.html#...