Web Gateway will offer both methods (NTLM and Basic). It is up to the browser or client to choose what type of authentication it will use (NTLM or Basic).
Most modern browsers will choose the most secure first (so NTLM). If it fails, it sometimes will fall back.
In MWG 6 it is not possible to enable (or simply offer) Basic authentication on a per client basis. In MWG 7 it is.