Unfortunately, work queues aren't a great abstraction for serialisisg access to a shared resource. Given that all iOS devices so far have had a single CPU core, most uses of threading will have been for things like network I/O, audio, or running larger computations in a non-UI-thread, rather than dispatching work items to many CPUs.

Furthermore, GCD has only been around since iOS 4.0, so if your app needs to be compatible to 3.x, you can't use it.