The segue is defined in a storyboard, when instantiating the class manually with alloc and init, the controller has no reference to the IB object, therefore no segue is defined. You want to use the storyboard itself to instantiate your controller:
UIStoryboard *storyboard = [UIStoryboard storyboardWithName:@"Main" bundle:nil];
GamePlay1 *controller = [storyboard instantiateViewControllerWithIdentifier:@"GamePlay1"];
[controller end];
Please note that this is not the proper way of doing a segue, you need to obtain a reference to the current instantiated viewcontroller instead of creating a new one.