GCD is a mechanism to let one central authority dispatch threads across multiple cores, for all running applications (including the OS).
This is what most people talk about, and what is most obvious from the name, but it is not the interesting part of GCD.
The interesting part of GCD is blocks and tasks, and it is useful to the extent which it makes expressing parallelism more convenient to the programmer.
The "central management of OS threads" is marketing speak for a N-M scheduler with an OS wide limit on the number of heavyweight threads. This is only useful because OS X has horrendous per-thread overhead. On Linux, for instance, the correct answer is usually to create as many threads as you have parallel tasks and let the OS scheduler sort it out. Other operating systems (Solaris, Windows) have caught up to Linux on this front, but apparently not OS X. If you can get the overhead of OS threads down to an acceptable level, it is always better to avoid multiple layers of scheduling.