This page is composed of notes when using the GNU General Public License for non-software projects especially creation processes generating physical objects or tangible artistic works.
DISCLAIMER : There is a fundamental difference between a physical object and non-physical object (like software or a stream of bits): the capacity of duplication. Free software is actively relying on that duplicability process to ensure the freedom to the users. Applying a free software license (e.g. GNU General Public License) to a physical object is not the objective and would not make much sense.
The primary objective is to protect the freedom around the creation process of creating the physical objects themselves. In that scope, the creation process is an activity of the mind covered by the "protection" of literacy and artistic works. As the creation process is a suite of instruction just like any computer program or cooking recipe, we can apply free software licenses to the description of the process to create a physical object.
Industrial design right is a different kind of legal right that could be sometime conflictual with copyright/"droit d'auteur". Some European countries don't allow to have a cumulative application of the copyright/"droit d'auteur" and the industrial design right.
To avoid any potential issues when releasing process to generate physical objects (that could be potentialy covered by the "industrial design right") under a free software license, we recommend to only use the application of the free software license and avoid any application for industrial design right.
It's a different law covering the specific aspects of topography from an integrated circuit. If the work can be covered by copyright/author-rights, we recommend to only apply the GNU GPL on those original descriptions generating the topography.
As the license is only covering the process description to create the work, the resulting object or design is not covered by the free license. So it's not mandated to put the license statement on the resulting object. You are free to reference the license and information about the process to create the object but this is not a legal requirement.
If you want to show that the process behind the object is free, you can still put a reference. The reference can be a bar-code or QR code pointing to the "source code" or the design process information along with the free license.
It's easy to modify a software, as this is composed of modules or functions that you can easily read and understand. For a creation process, this is not always obvious. We highly recommend to split or modularize the building process or design process to ease contributors to improve the process and add easily section in a process to create physical objects. This is just like software, if you can easily read the process, you can have more contributors to your project.