Gmail accounts can be synced using either IMAP/SMTP or the Google API methods. The Google API (OAth) is preferable.
Here are the simple steps.
okay here are the brief instructions:
click on email.
Delete the current account if needed (in case it was synced with IMAP): go to the top right and click the Red "X"
Next - add the account anew. Click the little portrait and Add new account.
Select Gmail New API Sync (leave all fields blank)
click on Authorize with Google and follow familiar prompts
Lastly, select all the folders you want synced and click Add/Save. All the main Google folders are required Inbox, Sent Mail, Drafts, Trash.
It may take a while for all emails to sync depending on the number of folders and messages you sync. Initially, we only go back about 60-90 days.
I hope this helps.