Please follow the below steps to configure your email.
> Open Lightroom.
Click on File > Email Photo
Click on the drop down box for From
Choose Email Account Manager
Select the current configuration for Gmail in the left hand side panel
Click on Remove
Then click on Add.
Type in the Account name and choose service provider as Gmail from the drop down list.
Click on OK.
In Credentials Settings, type in Email address, Username and Password.
Once it is validated, you should be able to send email using your Gmail account from Lightroom.
Please post a screen shot if you get any error message.