Enable your Gmail account for IMAP
Internet Message Access Protocol (IMAP) is a protocol that allows you to download messages from a mail provider's servers, such as those for Gmail, onto your computer so you can use Microsoft Outlook to view and edit your email, even when aren't connected to the Internet.
Enable IMAP on for your Gmail account
To make your Gmail messages accessible by Microsoft Outlook, you need to enable it for IMAP.
Sign in to your Gmail account by using a browser that is supported (Google Chrome, Firefox, Internet Explorer, or Safari).
Choose or click the gear icon () on the top right.
Choose Settings > Forwarding and POP/IMAP.
Select Enable IMAP, and then choose Save Changes.