GPL is a license designed primarily for software, but it can be used for non-software. The license text has many mentions of "source code", which is ambiguous for non-software.
GNU/FSF claim that GPL can be used for non-software, "as long as it is clear what constitutes the “source code” for the work".
The GNU GPL references the “source code” of a work; this “source code” will mean different things for different kinds of information, but the definition of “source code” — provided in the GNU GPL — holds true in any case: “The source code for a work means the preferred form of the work for making modifications to it.”
(bold is mine)
Unfortunately, no concrete examples are given.
Using common sense, I interpret this "preferred form" to mean for example PSD/XCF files for images, and DAW project files and/or MIDI files for audio/music. However, life experience has taught me that law and common sense seldom overlap.
Are there any notable examples of GPL being used in this fashion - to preserve access to the project files? Or any authoritative persons clarifying this issue?
Furthermore, unlike software, non-software can often be manipulated without "source code", which raises this hypothetical: what if I take a lone, GPL-licensed image file, create a derivative using Photoshop, am I required to share the PSD file under GPL, in addition to the derived exported image?