1) You shouldnt modify index.php
2) In addon desc I made error. You should use <GRAVATAR_COMMENTS> tag instead of <GRAV_IMAGE_COMMENTS> (I finished adding on PP page this addon 15 minutes before I left home for weekend - rush was a reason of error). Sorry for that.
3) Default gravatar is set to (line 55 of /addons/gravatars_comments.php):
$def_gravatar = 'email@example.com';
It is used for posts without gravatars as default. For rest of comments (with emails) it tries to use commenter email.
4) Did you also put content from /includes directory to your's place??