Hi
Firstly I would recommend taking a look at the our knowledge base articles on WinGate configurations in the Active Directory.
http://support.qbik.com/index.php?_a=knowledgebase
Here you will find that your client machines should have their DNS settings pointing to the AD DNS server so that Active Directory can function properly.
On the AD DNS server the "forward lookup zone" setting in the DNS server(that the server uses to resolve Internet requests) must be pointed at the Internal IP of the WinGate machine. WinGate will then handle all of the Internet requests and let the AD DNS server handle all the Active Directory requests.
The knowledge base articles will help you with the config you will need.
Erwin