I found this on the Workspace Marketplace but are wondering how to use it after it’s been installed in my Workspace Organization!?
https://workspace.google.com/marketplace/app/hubspot_gemini_for_google_workspace/584788731981
Hi @CEP-MUC and welcome, it’s a pleasure to have you here! 
Thanks for asking the HubSpot Community!
When checking the link you shared, I can see that this integration has been developed by Google, see screenshot below.
So the best next step here would be to contact Google Support to find out more about their integration.
Here is anyway an article I found about Gemini for Workspace that can be of interest to you.
Also, you might want to take a look at this: “How to Do 10 Hours of Research in 20 Minutes: The Marketer’s Guide to Google Gemini & NotebookLM”.
Thanks and have a great day! 
Bérangère