I was just at the GB a few weeks ago (mid-February), and I have to agree regarding the Roof Lounge. The food was wonderful - fresh fish of the day wonderfully prepared, excellent and attentive service, and a very comfortable setting - but, the price was well-above what I thought was reasonable for Athens, even at a property like the GB.
You should ABSOLUTELY go upstairs, camera in hand, and go out on the terrace. You'll have some spectacular views of Syntagma (Constitution) Square, the Parliament building, the old 1896 Olympic stadium, and, of course, the Acropolis. Enjoy!