> the argument would be that technically the GPL says you should be able to get source code for the program from Y, even though you didn't buy their product P - you were harmed by their refusal to do what the GPL requires, so you can sue them.

I don't think that's the case. Whenever I see discussion about GPL violations, the copyright holder is the one who has to go after the violator (and often getting them to do something is difficult, because legal challenges can be expensive); the consensus seems to be that users who receive the software in a manner not compliant with the GPL don't have standing to sue. I'm not sure if a user has ever tried, though, so not sure if this has been tested in court.