the earliest publicly discussed major GPL violation
was by NeXT computing when Steve Jobs attempted and
failed (thanks to RMS' GPL enforcement work) to make
the Objective C front-end to GCC proprietary.
There's a little more information about that exchange at http://clisp.cvs.sourceforge.net/viewvc/clisp/clisp/doc/Why-... (Stallman brought it up when enforcing the GPL on another free software project: "They had wanted to distribute the Objective C parser as a separate proprietary package to link with the GCC back end, but since I didn't agree this was allowed, they made it free.").
It sounds like Stallman wasn't entirely sure the GPL would have prevented Jobs from doing what he wanted, but was convinced by a lawyer that the FSF would have a strong case ("It seemed to me at the time that it was [legal] ... but since the result was very undesirable for free software, I said I would have to ask the lawyer. What the lawyer said surprised me; he said that judges would consider such schemes to be 'subterfuges' and would be very harsh toward them. He said a judge would ask whether it is 'really' one program, rather than how it is labeled.").
