A: If you get an HTML form asking you to enter username, password, and database, use what your gnats administrator has told you. In this case, your authentication data is checked with gnatsd's own user database.
If your browser gives you a window asking for username and password (or for "credentials"), the authentication is done by the WWW server. You should have got the necessary data either from your gnats administrator, or from the webmaster of the site offering Gnatsweb access.