Use the theme's avatar as the default if Gravatar doesn't have one.

modules/comment/views/comment.html.php

@@ -2,7 +2,8 @@
 <li id="gComment-<?= $comment->id; ?>">
   <p class="gAuthor">
     <a href="#">
-      <img width="40" height="40" src="<?= $user->avatar_url(40) ?>"
+      <img width="40" height="40"
+           src="<?= $comment->author()->avatar_url(40, $theme->url("images/avatar.jpg", true)) ?>"
            class="gAvatar" alt="<?= $comment->author_name() ?>" />
     </a>
     <?= t("on ") . date("Y-M-d H:i:s", $comment->created) ?>