We just released a new version of the Google+ for iOS SDK (2-Oct-2013 v1.4.0) that now allows for your app to use native sharing so that your users do not need to leave your app to share content to Google+.
The following modifications to your code will enable the new in-app sharing experience:
if ([[GPPSignIn sharedInstance] authentication]) {
id<GPPNativeShareBuilder> shareBuilder = [[GPPShare sharedInstance] nativeShareDialog];
[shareBuilder setURLToShare:[NSURL URLWithString:@"sample valid url"]];
[shareBuilder setPrefillText:@"sample prefix text"];
[shareBuilder open];
}
For more information on this feature, see Sharing to Google+ from your iOS app.
This isn't exactly what you asked for, but I think its an even nicer experience!