This is one of the one-time promo pop-ups that this extension doesn’t work on*, and I find those annoying too. It will, however, hide nearly all Gemini-related menu items, commands, and search features.

* This is because when a web page creates an internal overlay, often it disables access to the content behind it in addition to covering it up. This means hiding the popup would make it impossible to use the page itself. This extension doesn’t use active code to be more trustworthy, so there’s not a good way to remove those pop-ups.