Earlier you should have an AIM account to chat with your AIM friends in Gmail chat. Therefore if you do not sign in to AIM from Gmail you can not chat with your AIM friends. Now you do not want an AIM account. You can directly chat with them.
How to chat with AIM friends in Gmail chat?
Simply type their AIM screen name in chat “search, add or invite” filed. Then choose “invite to chat” from the popup menu.
Once your AIM friend accepted your invitation you can chat with him/her in Gmail chat.