GPL violations as far as I understand it are always a matter to be settled between the original authors and the offenders. I am not aware that the GPL gives bystanders and angry fanboys'n girls the right to sue anyone.
The author(A) allows the distributor(D) to distribute their(A) code so long as they(D) provide the source plus any modifications to the recipient.
The recipient has no right to demand the code due to there being no agreement to provide it between the distributor and the recipient.
So the author, on behalf of the recipient, must be the one to enforce the conditions of the agreement they made with the distributor.
In cases involving projects like Linux or BusyBox this can be anyone who has committed code, as these projects do not require copyright assignment.
In cases involving projects like GNU and Ubuntu all committed code is owned by the parent organization, the FSF and Canonical respectively. So they are the only ones who can enforce the GPL.
That said, the end user can report violations to groups like The Software Freedom Law Center and GPL-Violations.org. These groups have contacts to various copyright holders and will assist with enforcing them on a pro bono basis.