go to GTMOAuth2Authentication.m file method setKeysForResponseDictionary in dic returns access token at end of method.
accessTocken = [dict valueForKey:@"access_token"]; // access tocken pass in .pch file
[accessTocken retain];
and in your controller
- (IBAction)momentButton:(id)sender {
NSString *str = [NSString stringWithFormat:@"https://www.googleapis.com/oauth2/v1/userinfo?access_token=%@",accessTocken];
NSString* escapedUrl = [str stringByAddingPercentEscapesUsingEncoding:NSUTF8StringEncoding];
NSURL *url = [NSURL URLWithString:[NSString stringWithFormat:@"%@",escapedUrl]];
NSString *jsonData = [[NSString alloc] initWithContentsOfURL:url usedEncoding:nil error:nil];
NSMutableDictionary *proDic = [[NSMutableDictionary alloc] init];
proDic=[jsonData JSONValue];
NSLog(@"%@",proDic);