Many already know you can add your Gmail account to Outlook. Doing so has many advantages that some find helpful. Setting it up can be a frustrating experience if you do not take some preliminary steps.
First is to make sure you have the correct username, (email address) and password. The next step is to access your Gmail account online. Go to the gear in the upper right corner and click the option for Settings. In Settings click on the blue link, at the top, titled Forwarding and POP/IMAP. About half way down the page look for IMAP Access and Enable IMAP. Click Save Changes at the bottom of the page.
Next Click on the User icon in the upper right corner. If your name is Andrew, that icon would be an A. Click it and then select My Account. Click Sign In and Security. Then click ON the option for Allow less secure apps: Without that option selected Outlook will not process the Gmail e-mail. Exit your Gmail account.
Open Outlook and finish setting up your Gmail account. Done.