It sounds like Grand Central is the mechanism through which multitasking roles are described/created; i.e. programmers declare the background parts of their app as closures (GCD blocks) which are handed off to a task manager.

I might be wrong on this.

none. grand central is a scheme to make it easier to parallelize your code, generally so it can take advantage of two or more cpu cores. not much use on iphone devices, which aren't going to have more than one core for a long time. blocks add closure-like abilities to C. multitasking, in this context, is a way to run two or more third-party apps at once. so, basically, no overlap at all.