I guarantee you will have to spend money to use Gaikai. Either it will be another tier above PS+ or you will have to buy your PS3 games again. They aren't just going to let you play your old PS3 downloaded library for free through a streaming service they paid half a billion for. That's why you keep your PS3.
You didn't get my point. I was talking about the PS3 Plus library, which you got with the Plus subscription. I guarantee that they'll find a way to make those games available to you on PS4.