Can't Authenticate my work gMail — I get right through the OTP dance then it simply fails
mitch   [1:20 PM]
This happens to me also. I have found a workaround.

WORKAROUND:

In gmail, go to My Account (from your user icon), then Sign-in and Security, Allow less secure apps must be turned on.

Then, if you don't know your actual gmail password to use: In OneLogin, on the App Home page, click the Pencil icon next to the New App button top right, then click the icon you use to access GMail. In the resulting pop-up window, click the Change Google Apps password link and enter a password.

Now, go to System Preferences -> Internet Accounts, Add Other Account..., then configure gmail with plain ole IMAP (imap.gmail.com, smtp.gmail.com). PRESTO! Gmail working in Mail.app.