First, if you unchecked any IMAP options on the gmail website, please go back and make sure they are re-enabled.
Next, it will probably be easiest to remove and then re-add your gmail account to your Mac's Mail app.
Click "Mail" in the menubar and choose "Preferences"
Choose the "Accounts" tab
Select the Account you wish to delete
In the lower left corner of the "Accounts" Preferences window, click the - sign.
Once the account has been removed, in the lower left corner of the "Accounts" Preferences window, click the + sign.
Choose your email service name (Google or gMail) and click Continue.
On the "Add a Mail Account" window, type your name in the top box, your complete email address in the middle box (full email address), and your email password ***** the last box.
After a moment or two, the account should be configured and enabled.
If you receive errors when creating the account, double-check your email address and password. If issue remains, reply back for further troubleshooting.