Thanks for the comments, guys!
@Charybdis:
As Tempest has already pointed out, the stucture is the enlarged aircraft hangar which was fitted to "Gneisenau" during her stay in Brest in late 1941.
Up to three Arado 196's could be carried after it's installation. Interestingly, the catapult to launch the planes was atually mounted inside of the hangar.
I have made a quick drawing to show it's functionallity: