I have the same problem with Gewirth's argument that a lot of philosophers have had. I just don't see how he can get from step two to step three. In step two he says that the agent must see freedom and well-being as necessary conditions for any of his purposes. I'm OK with that. He then says that the agent must hold that he "must" have freedom and well-being. In a sense, I can agree with that too. But then he goes on to say that the agent must hold that he has rights to freedom and well-being. That's the step I find hard to accept. 

It seems to me that someone can believe that she must have X in order to do something  without believing that she has a right to X.

I know that Gewirth has considered this objection and believes that it is mistaken, but I just don't see how he can make the inference he makes.