Typical use case for GCD

GCD

// Doing something on the main thread

dispatch_queue_t myQueue = dispatch_queue_create("My Queue",NULL);
dispatch_async(myQueue, ^{
    // Perform long running process

    dispatch_async(dispatch_get_main_queue(), ^{
        // Update the UI

    });
}); 

// Continue doing other stuff on the 
// main thread while process is running.