Everybody whose rights were infringed. The GPL often technically makes that "everybody else" by granting what were otherwise exclusive rights (to make and distribute copies) to everybody and then taking them away from infringers.

So e.g. Company X makes a GPL'd program to do A, but Company Y just copy pastes it into famous product P and acts as though they made it and obviously doesn't give out source. As a random person who doesn't even own P, 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.

Now, suing is probably not a good idea in this case, a court is likely to either insist you aren't really injured or that they can't help you, or both, but I think it could work at least in theory.