ok, i now can give some answers myself:
Is it somehow possible to get a 360 degree angle-of-view with the GEM camera inside a framebuffer?
I just noticed, that the camera clips at 180deg (using the "perspec" message). That means, outside 180deg nothing but the background color is rendered.
The camera in GEM is an "ideal" camera, which has no angle, but only a size. So it is technically not possible to get more than 180 degrees.
I then tried to build a virtual multi-camera system using two framebuffers with 180deg camera angle each. But apparently the camera direction inside the framebuffer is fixed and can not be set. Or can it somehow?
The same result can be achieved by rotating the content.
So, my new multi-camera approach is as follows: I connect my content to three framebuffers (At the second one, everything gets rotated by 120deg, and at the third one by 220 deg). Then I stitch these three parts together to one continuous video using Extended View Toolkit.
My new question: Isn't there some easy way to do the same thing?