Lets think about this.
Heres what the argument amounts to. Let P be some action of mine in the future.
(1) Necessarily, God knows that I will Q.
(2) Necessarily, If God knows that I will Q, then I will Q.
(3) Necessarily, 'If I will Q, then it is not possible that not Q'
(4) Necessarily, If it is not possible that not Q, then I can not Q freely.
(5) Therefore, necessarily, 'If God knows that I will Q, then I cannot Q freely.'
First thing to notice is that (2) follows from(1) only because knowledge is factive. That is, because knowledge requires truth. So the argument is not specific to theism. Anyone who believes that there is a truth about the future will have this problem.
If you come to believe that there is no truth about it, then it seems that it should be no problem for God. Omniscience means knowing all truths, and if there is nothing for God to know, well, not his fault.
Second, there is a notorious problem with the move from (3) to (4). A technical point first. There is a mixing of modalities, and it is not clear that the argument is valid. It does not seem to be necessary in that broadly logical sense that if I will do something that I could not have done something else. Rather there is some sort of relitivized necessity here. Some sort of closedness from causal change. But the past is closed to me, and yet I do not regard my past actions as unfree. I cannot change any of my past actions, unfortunatly, yet I did them freely.
Furthermore, there seem to be counterexamples to the idea that Free will means, 'could have done otherwise'. See Harry Frankfurt's counterexamples.
On the other hand, if to be free in regard to some future free action Q is simply for the possibly not Q in the broadly logical sense to be true, then there is no incompatibility between these two statements: 'I will Q', and 'I will freely Q'. For it can be true that in fact I will Q and that insome possible world I did not.
The bigger lesson here, I think, is that we just do not understand free will very well at all. Free will is a problem for everyone. And so I think we ought to be careful about launching arguments, one way or the other, that depend on their soundess on such a obscure and illusive concept.