The short answer: It's complicated. But to give answers at face value: 1. Unfortunately, in most cases, nothing. GPL violations are rampant, and usually withou

The short answer: It's complicated. But to give answers at face value: 1. Unfortunately, in most cases, nothing. GPL violations are rampant, and usually without serious consequence for the infringer. 2. Like any other copyright violation, through the courts. But there is a history of successful GPL enforcement by the mere threat of going to court (since what you're usually after is compliance, not damages). 3. Depends heavily on the jurisdiction, but in most of them, it certainly helps to have a copyright holder involved (I would go as far as to say it's a necessity). But you also need money, so most likely, something like the EFF or SFLC.