From what I understand the GPL license can only be enforced by the copyright holder.
Therefore, given a scenario like this:
A developer finds an abandoned software project licensed under GPL.
S/he builds upon the project and extends the functionality (which I'm assuming would counts as derivative work).
S/he releases the improved source code to everyone else.
At some point the GPL license has been violated.
Could the new developer enforce the license if the original copyright holder is long-gone or undisclosed in the original work?
Or is the license only enforceable by the original creator?