I don’t think this is something that can be configured from an email sender (Ghost / mailgun / etc) - quick googling suggests you should set up gravatar for your email account, but I don’t know if that actually works.
The email address (“from” which the newsletters of my Ghost site / any Ghost template site? go out) set in the Members settings is not technically a ‘real’ email address one would have access to but is rather a no-reply address, you wouldn’t be able to setup a gravatar for it.
Mailgun supports receiving and doing minor processing for incoming mail (though you might have to be on an explicilty paid plan). If not mailgun, you can always set up a temporary email service (like google apps, zoho, etc.), get the gravatar email sent to you, and then remove it. You can have as many email senders as you would like, you’re just limited to one receiver.