The Registration Token is given to the registration handler from didRegisterForRemoteNotificationsWithDeviceToken
All code below is taken from the GCM Sample from Google.
First, declare a handler in your application:didFinishLaunchingWithOptions:
_registrationHandler = ^(NSString *registrationToken, NSError *error){
    if (registrationToken != nil) {
        weakSelf.registrationToken = registrationToken;
        NSLog(@"Registration Token: %@", registrationToken);
        NSDictionary *userInfo = @{@"registrationToken":registrationToken};
        [[NSNotificationCenter defaultCenter] postNotificationName:weakSelf.registrationKey
                                                            object:nil
                                                          userInfo:userInfo];
    } else {
        NSLog(@"Registration to GCM failed with error: %@", error.localizedDescription);
        NSDictionary *userInfo = @{@"error":error.localizedDescription};
        [[NSNotificationCenter defaultCenter] postNotificationName:weakSelf.registrationKey
                                                            object:nil
                                                          userInfo:userInfo];
    }
};
Call your handler in the application registration callback:
- (void)application:(UIApplication *)application
didRegisterForRemoteNotificationsWithDeviceToken:(NSData *)deviceToken {
    // Start the GGLInstanceID shared instance with the default config and request a registration
    // token to enable reception of notifications
    [[GGLInstanceID sharedInstance] startWithConfig:[GGLInstanceIDConfig defaultConfig]];
_registrationOptions = @{kGGLInstanceIDRegisterAPNSOption:deviceToken,
                         kGGLInstanceIDAPNSServerTypeSandboxOption:@YES};
[[GGLInstanceID sharedInstance] tokenWithAuthorizedEntity:_gcmSenderID
                                                    scope:kGGLInstanceIDScopeGCM
                                                  options:_registrationOptions
                                                  handler:_registrationHandler];
}
The GCM token to use is simply the "NSString *registrationToken" in the registration handler.