The GNU General Public License is a free, "copyleft" license for software and other kinds of works. As the most widely used "copyleft" software license, it was designed to allow end users freedom to use, study, share (copy), and modify software. Derivatives of GNU GPL-licensed material can only be distributed under the same terms. Read the Quick Guide to GPLv3.
Isn't the GNU license intended for software?
Originally, yes - but it can be applied to the "source code" for any work. In the case of 3D prints, that applies to the digital 3D model.