I never used gravatar, and I don't have an account in there, but I'm not naïve and I know how gravatar is widely used and not having an "avatar ready" in registration is a thing of the past.
You'll not get an error. Since you don't have gravatar as I, after registration it would be as it is now you'll not get any avatar.
But if the email is valid for gravatar then it should use it. Like many other systems works nowadays. As wordpress for example.
This could be an extension ofcourse, BUT since phpbb already have an gravatar on/off at ACP the only thing it makes sense is to really make use of it properly, if not, then get rid of it! No need to confuse admins or making them think phpbb is weak or buggy when it is only a old guy that is afraid of the future.