From GPLv3 Wiki
One person's experiences
There are comments from my own personal experience of GPL Violations:
When requests are ignored/denied, you have no legal enforcement remedy.
When you find someone stealing GPL code, and ask them for the source, and they either refuse, give you something bogus, or give you isolated useless fragments without usage instructions - there is nothing *you* can do about it.
The only way to actually compel someone in violation is to convince one of the original authors of the source code to sue them. If authors are even contactable, or still exist at all, this is an almost impossible task.
I wish to make the following suggestion to remedy this problem.
The FSF people obtain a copyright/trademark on a particular phrase of words - for example - "This Software is GPLvX protected(tm)". GPL authors are asked to include this phrase in their source and (where practical) in their object and executable output.
Then - when violations are encountered, there is now additional legal action that can actually be taken.