Just generate the URL yourself. It’s just a hash of the user’s email address.
function get_gravatar_url( $email ) {
$hash = md5( strtolower( trim ( $email ) ) );
return 'http://gravatar.com/avatar/' . $hash;
}
This function requires that you pass the user’s email address in … but you could do anything you need to programatically grab the user’s address.