Step 1: Confirm that Gopher for Chrome is an allowed app on your domain, and that your user accounts are authorized to install it.
As a Google Workspace super admin, under Apps > Marketplace apps > Settings for Marketplace Apps> Manage access to apps, confirm that users in your domain are allowed to install allowed apps from the Google Workspace Marketplace.
Next, allow Gopher for Chrome within the Google Marketplace Apps settings. To do that, navigate to Apps > Marketplace apps > allow listed Marketplace apps. Select the + icon in the right-hand corner and search Gopher for Chrome. Check the box next to our add-on and select Allow list, then Confirm.
Step 2: Install the Sheets Add-on from the Google Workspace Marketplace
In Google Sheets, start by going to Add-ons > Get Add-ons. Search for Gopher for Chrome and select Individual Install.
Or, cut to the chase and install it directly from here.
Once installed, you can open Chrome Gopher from any Google Spreadsheet from Add-ons > Gopher for Chrome > Launch Sidebar
It is important that you do NOT install the Add-On domain-wide as it will appear for ALL users.