[pmjordan]

GCD is purely callback-based. It works neatly with Apple's "blocks" (inline capturing functions) extension to C, which make dealing with callbacks slightly nicer, but it's still just a sequence of callbacks.

Fully fledged coroutines let you save and resume execution in place. The main advantage is readability of conditional/looping control flow, which tends to quickly devolve into a mess in callback style code.