UPDATE: Please visit the Gmail Help Forum if you have any additional question.
Have you switched to the new Hangout chat and you want to revert to old Gmail chat? Then I have a solution for you. Recently, Google made major changes to their interface on Google+ which also affect several other products like Gmail chat.
Old Gmail chat
Below is a screenshot of the old Gmail chat what was included originally.
Try new Hangout chat
If you want to try the new hangout chat, click on your picture and then click “Try new Hangout chat”.
Revert to old Gmail Chat
After trying it, and if you don't like it for now, you can click on your picture again. Then choose “revert to old chat” to revert to old Gmail chat. Keep in mind that eventually the old chat will go away and will be replaced by the new one. There are future changes coming to it so be patient has this feature is being upgraded.