How server should authenticate on behalf of iOS user?

I'm using google-api-nodejs-client to get profile information and to login with Google. I want the iOS client to login to Google through my server.

- (void)finishedWithAuth:(GTMOAuth2Authentication *)auth error:(NSError *)error {
    if (error) {
        NSLog(@"Error: %@", error);

        abort();
    } else {
        NSString *serverCode = [GPPSignIn sharedInstance].homeServerAuthorizationCode;
        NSURLRequest * urlRequest = [NSURLRequest requestWithURL:[NSURL URLWithString:[NSString stringWithFormat:@"http://localhost:1337/auth/google/callback?code=%@",serverCode]]];
        NSURLResponse * response = nil;
        NSError * error = nil;
        [NSURLConnection sendSynchronousRequest:urlRequest
                                              returningResponse:&response
                                                          error:&error];
        NSLog(@"%@, %@",[response description], serverCode);


        [self refreshInterface];
    }
}

finishedWithAuth: returns the following error:

json={
    error = "invalid_audience";
    "error_description" = "The audience client and the client need to be in the same project.";
}}

What is the audience and how do we add the client ID to our server?

Thanks!