Does it have to be a gateway?

Could you install Samba on the Unix server, configured to authenticate against the Windows AD server? If the AD server is 2003, you can authenticate via LDAP using their normal domain user/password, and squash everything down to a single unix user/id. This isn't overly secure, naturally. Alternatively, you can configure the Samba server to authenticate via Kerberos with the AD server (2003 supports this, as well, 2008+ requires it). This requires the use of winbind, which provides mapping between AD users/IDs, and Unix, in such a way that the users do not need to have local accounts. I'm not sure if you can squash everything down to a single user, when configured like this -- at least, I never had any luck doing so.