Regular Gmail synchronization issues:
Updated: As of lately Google has started disabling IMAP by default so I will show you how to turn it back on.
First click on the settings>See all settings button.
From this page please click on Forwarding and Pop/IMAP and then make sure IMAP is enabled.
Let's make sure that your 2-Step verification login is off and Allow less secure apps is on.
To do this login to your Google account and then click on your profile in the top right then click on Google account.
From this screen click on security and then you will see both options to make sure 2-step verification is off.
And then at the bottom of the page turn on less secure apps.
Here is Video:
This should fix any issues that would be stopping your google account from syncing.
However, If you are still experiencing issues, especially the error message "web login required" visit this link to allow access to your Google account.
https://accounts.google.com/DisplayUnlockCaptcha
Here are some Google Articles on this issues as well:
Keeping 2-step Authentication on with App Passwords:https://support.google.com/accounts/answer/185833?hl=en
Instructions to enabling IMAP: https://support.google.com/a/answer/105694?hl=en
Less Secure Apps: https://support.google.com/a/answer/6260879
Another Article for IMAP: https://support.google.com/mail/troubleshooter/1668960?rd=1
G Suites Users:
If you are a G Suites user it is the same fix their menus are just set up differently.
First, you will want to navigate to admin.google.com and sign in. From here you will see this page please select security.
Now go ahead and select the option basic settings.
Then click on go to settings for less secure apps.
Now from this last page you will either select enforce less secure apps for all or allow the user to enable them. If you select allow the user to enable them please then follow the Gmail instructions above to finish the syncing.
support.google.com
Turn POP and IMAP on and off for users - G Suite Admin Help
If your G Suite users want to use desktop clients, such as Microsoft® Outlook®, Apple® Mail, Thunderbird®, or Outlook® Express®, to access their G Suite mail, you need to enable P
support.google.com
Control access to less secure apps - G Suite Admin Help
This article is for administrators. For actions users can take, see Less secure apps & your Google Account. You can block sign-in attempts from some apps or devices that are less secure. App
support.google.com
Use IMAP to check Gmail on other email clients - Gmail Help
Set up IMAP and change your SMTP settings to read Gmail messages in other mail clients, like Microsoft Outlook and Apple Mail. When you use IMAP, you can read your Gmail messages on multiple devices
Also, synchronizing Gmail in AUTOMATE - view article