ARC

After

#import "ViewController.h"
#import <CoreMotion/CoreMotion.h>
@implementation ViewController
{
    CMMotionManager *motionManager;
}

- (void)hoge
{
    NSLog(@"hoge");
}

- (id)initWithCoder:(NSCoder *)coder {
    self = [super initWithCoder:coder];
    if (self) {
        motionManager = [[CMMotionManager alloc] init];
        
        __weak ViewController *weakSelf = self;
        
        [motionManager startGyroUpdatesToQueue:[NSOperationQueue mainQueue] withHandler:^(CMGyroData *gyroData, NSError *error){
            
            __strong ViewController *strongSelf = weakSelf;
            if (strongSelf){
                [strongSelf hoge];
            }            
        }];
    }
    return self;
}

- (void)dealloc
{
    [motionManager stopGyroUpdates];
    NSLog(@"Dealloc");
}

@end

Before

#import "ViewController.h"
#import <CoreMotion/CoreMotion.h>
@implementation ViewController
{
    CMMotionManager *motionManager;
}

- (void)hoge
{
    NSLog(@"hoge");
}

- (id)initWithCoder:(NSCoder *)coder {
    self = [super initWithCoder:coder];
    if (self) {
        motionManager = [[CMMotionManager alloc] init];
        [motionManager startGyroUpdatesToQueue:[NSOperationQueue mainQueue] withHandler:^(CMGyroData *gyroData, NSError *error){
            [self hoge];
        }];
    }
    return self;
}

- (void)dealloc
{
    [motionManager stopGyroUpdates];
    NSLog(@"Dealloc");
}

@end