I have a Google Workspace account. As administrator you can disable Gemini for both the App and the Workspace.
To disable it for the Workspace requires that you disable it for each service in your Workspace.
If you have a starter or classroom edition, this functionality is not available, but Google support can make it visible.
As Administrator, click on Help, fight your way through the bot. I asked to "disable Gemini" and kept clicking "this does not help" when it came back.
Eventually you can start a chat with support.
It helps if you have a screenshot of the Gemini Workspace screen that shows there are no controls. Upload the screenshot into the chat.
The agent enabled the controls within a few minutes, so I could disable Gemini for each service. It's now finally gone from Gmail on the web and my phone.
Be prepared to be told that Gemini can help you with your job .. which I declined to engage with.
Source: I did this an hour ago after trying many times over the past three weeks.