The library may work, but Grand Central Dispatch, which is the whole point, is really an OS X thing. BSD may have adopted it, not sure. I'm pretty that confident Linux has not.
Grand Central Dispatch is just the marketing name for libdispatch.
The do use pthread work queues if available, which they are on OSX & BSD. On linux a thread pool is used
Yes, so there's absolutely no reason to use it if you're primarily targeting Linux.