To use your Gmail account click on the "my add-on accounts" link at the top of the admin console.
Choose "Gmail" from the drop down list.
Give the Add-on service a name (i.e. My Gmail Account)
Enter your Username and Password
Next, you need to set up your server(s) to use your Gmail credentials.
Begin, by clicking on "Install New" from the Add-On bar
On the next screen, choose "Gmail" from the list of available Add-Ons
Next, choose from your existing credentials
Lastly, press the green button that says "Install Add-On"
Now, your server is configured to send and receive emails via your Gmail account