Finally i found the solution . . .i think this will help anybody else Follow the below steps to integrate gmail with your application .

1.Add following classes to you project .

GTMHTTPFetcher.h , GTMHTTPFetcher.m ,GTMOAuth2Authentication.h, GTMOAuth2Authentication.m,GTMOAuth2SignIn.h,GTMOAuth2SignIn.m,GTMOAuth2ViewControllerTouch.h,GTMOAuth2ViewControllerTouch.m,GTMOAuth2ViewTouch.xib,SBJSON.h , SBJSON.m

you will get these classes here : https://github.com/jonmountjoy/Force.com-iOS-oAuth-2.0-Example

Note : if you are working under ARC Environment then you have to disable the ARC for following files :
GTMHTTPFetcher.m , GTMOAuth2Authentication.m , GTMOAuth2SignIn.m, GTMOAuth2ViewControllerTouch.m

To disable ARC for source files in Xcode 4, select the project and the target in Xcode. Under the target "Build Phases" tab, expand the Compile Sources build phase, select the library source files, then press Enter to open an edit field, and type -fno-objc-arc as the compiler flag for those files.

2. add the following frameworks

security.framework , systemConfiguration.framework

3. Register your app to google api console …. here : https://code.google.com/apis/console

Then go to ApiAccess section , create client id for iOS app . then you will get clientID, ClientSecret and RedirectUrl

*4. Now it's time for coding . . . .*
create a signIn button in your controller and set the action for that . Here when the user click the button SignInGoogleButtonClicked method gets called .

//import GTMOAuth2Authentication , GTMOAuth2ViewControllerTouch

#define GoogleClientID    @"paster your client id"
#define GoogleClientSecret @"paste your client secret"
#define GoogleAuthURL   @"https://accounts.google.com/o/oauth2/auth"
#define GoogleTokenURL  @"https://accounts.google.com/o/oauth2/token"

-(void) SignInGoogleButtonClicked
{

 NSURL * tokenURL = [NSURL URLWithString:GoogleTokenURL];

    NSString * redirectURI = @"urn:ietf:wg:oauth:2.0:oob";

    GTMOAuth2Authentication * auth;

    auth = [GTMOAuth2Authentication authenticationWithServiceProvider:@"google"
                                                             tokenURL:tokenURL
                                                          redirectURI:redirectURI
                                                             clientID:GoogleClientID
                                                         clientSecret:GoogleClientSecret];

    auth.scope = @"https://www.googleapis.com/auth/plus.me";

    GTMOAuth2ViewControllerTouch * viewcontroller = [[GTMOAuth2ViewControllerTouch alloc] initWithAuthentication:auth
                                                                                                authorizationURL:[NSURL URLWithString:GoogleAuthURL]
                                                                                                keychainItemName:@"GoogleKeychainName" delegate:self
                                                                                                finishedSelector:@selector(viewController:finishedWithAuth:error:)];

    [self.navigationController pushViewController:viewcontroller animated:YES];

}



//this method is called when authentication finished

- (void)viewController:(GTMOAuth2ViewControllerTouch * )viewController finishedWithAuth:(GTMOAuth2Authentication * )auth error:(NSError * )error
{

    if (error != nil) {

        UIAlertView * alert = [[UIAlertView alloc] initWithTitle:@"Error Authorizing with Google"
                                                         message:[error localizedDescription]
                                                        delegate:nil
                                                        cancelButtonTitle:@"OK"
                                                        otherButtonTitles:nil];
        [alert show];
    }
    else
    {

         UIAlertView * alert = [[UIAlertView alloc] initWithTitle:@"Alert !"
                                                         message:@"success"
                                                        delegate:nil
                                                        cancelButtonTitle:@"OK"
                                                        otherButtonTitles:nil];
        [alert show];

    }
}