Still waiting for Google Chat for Sidebar Messengers.
But for all those who can't wait, I found a solution to add it to your sidebar without any extension.
So, first of all go to: opera://flags/#sidebar-site-panel
Enable the flag and re-launch the browser.
Then, on then sidebar, you can add any sites you want.
Right click the sidebar --> hit "Sidebar Setup" --> scroll down to the bottom and it will be under "Custom site panels"
There you cann add the url: