Anton,
For this reason I am interested in GPL-ed real time operative systems.
you should perhaps take a look at Fiasco, which is an L4V2 kernel with the focus on real time properties.
I read the specification of the L4.Sec system, but unfortunately I am not able to fully understand the concept of capability space.
A capability in L4.sec is a references to a kernel object and permissions to use it. Kernel objects are for example threads, endpoints or tasks. Permissions are whether it is allowed to send to an endpoint or to modify the state of a thread.
A capability space is a table of capabilities. These capabilities are referenced through task local numbers called capability id's. This is very similar to file descriptors in unix.
If you need some pictures to understand this, have a look at my thesis, where I give a short introduction into L4.sec. You can find the thesis at http://os.inf.tu-dresden.de/papers_ps/kauer-diplom.pdf
Bernhard