Has there been an update to ElCapitan recently? If so you could check to see if mail settings have been affected.
Try going to the Account window, under SMTP setting select "Edit SMTP" - select your account from the list - under the advanced tab check the port is the recommended one, 465, - if your server requires SSL tick that option, if it requires TLS then port 587 is recommended - make sure that user name and full e mail address are entered correctly - add a password if it is blank.
If for some reason Outgoing Mail says Google is offline and greyed out preventing editing you could try.
Go to the "Mail" Preferences->Accounts Tab
Select the account on the left side,
Click on "Advanced" tab along the top right.
Deselect the box "Automatically detect and maintain account settings".
Select the General icon up top or another account after making the change. This will force the Mail app to prompt you to save the chnages.
After that you should be able to edit the account. If not, in SMTP window remove Gmail's server then add it back.
Account information tab: smtp.gmail.com
Advanced: Port 465 check use SSL
Enter your email and password. You might need app specific password if you have two step.
Assign to the Gmail account.