Half a day for the forbidden city si enough, so you can spend one day for the GW.
For the forbidden city it's NECESSARY to book the visit through a website by paypal. I don't remember the website but you can easily find it. If you don't book in advance, be sure to be at the ticket office of the FC very early. In august, at 8.30am it was already closed, but i booked the last ticket of the day during my last day in Beijing.
For the GW i went to Mutyanyu and i highly recommend it to you. Few people and there is also a part of the wall not restored, so you can see both. If you have a day you can go there by bus from Donzgzhimen, if i remember well