By default, Outlook deletes incoming messages from the server after they've been downloaded. It works that way for all POP3 email servers, not just Gmail. Otherwise the same emails would get downloaded to your Outlook inbox over and over again.
For corporate email using Exchange Server, their users do leave messages on the server but they have a "synchronisation" feature which avoids the problem of duplicates in their inboxes.
To prevent incoming messages being deleted from the server, open your Outlook email account properties, click More settings, click the Advanced tab, tick leave a copy of messages on the server, click 'OK'.