Hit Ctrl + T to open up a new tab.
In the address field of the new tab, type:
opera://flags/#sidebar-site-panel
and press enter.
That will take you to the flags page with the "Custom user sites for browser sidebar" flag scrolled into view and its name highlighted.
Click the drop-down at the right of it and change it from "Default [D]" to "Enabled".
Then, click the "Relaunch" button that appears at the bottom right of Opera.
Once Opera starts, click the 3 dots at the bottom of the sidebar, scroll down to "Custom Site Panels", click "Add custom site panel", click the text field in the "Use site as panel" dialog that pops up, type:
https://gemini.google.com/appand click "Create" or press enter.
