Well, Roundcube needs a mail server to show messages from. If you installed roundcube somewhere, you edited the roundcube config. There were lines like imap server, smtp server. I guess you have filled in there localhost. So the mail server may be on the same machines, as roundcube web application. in that case, you just need to know the external IMAP server address, SMTP server address and write that information to outlook.
Roundcube actually can check and show Gmail's mail