Not specifically gay, just a regular German B&B, but gay-friendly (as is most of Berlin), and close to many gay spots you may wish to visit. I enjoyed my stay there years ago, and would return on my next visit. For some weird reason, I recall having had the most peaceful, restful night's sleep while staying there, which, I know, isn't necessarily a good thing, , but I'm being a little serious here...

http://www.am-savignyplatz-hotel.de/