Let me guess your using a separate app for Gmail and the built-in email function for outlook.com?
If so you can add gmail via the built-in email functions and sync your calendar + contacts.
I think this has to do with enabling 2-factor authentication on my Google account, but the Mail app doesn't know how to prompt me for an access code. Dose Windows Phone 8.1 fix this?
Never mind, I generated an app password for this.
I already said it in the subject title. Google is a linked email account, but I was never prompted for the verification code like I was for Microsoft account.
Yes app password resolved it.