Step 1: Confirm that Gopher for Gmail is a whitelisted app on your domain, and that your user accounts are authorized to install it.
As a G Suite super admin, under Apps > Marketplace apps > Settings for Marketplace Apps> Manage access to apps, confirm that users in your domain are allowed to install whitelisted apps from the G Suite Marketplace.
Next, whitelist Gopher for Gmail within the Google Marketplace Apps settings. To do that, navigate to Apps > Marketplace apps > Whitelisted Marketplace apps. Select the + icon in the right hand corner and search Gopher for Gmail. Check the box next to our add-on and select Whitelist, then Confirm.
Step 2: Install the Sheets Add-on from the G Suite Marketplace
In Google Sheets, start by going to Add-ons > Get Add-ons. Search for Gopher for Gmail and select Individual Install.
Or, cut to the chase and install it directly from here. Note: This G Suite Marketplace link will most likely only work from an administrator account on your domain.
Once installed, you can open Chrome Gopher from any Google Spreadsheet from Add-ons > Gopher for Gmail > Launch Sidebar
It is important that you do NOT install the Add-On domain-wide as it will appear for ALL users.
Step 3: Perform the one-time client name and scopes installation for your domain
Because Gopher for Gmail performs very powerful actions on behalf of G Suite users, the sidebar (see below) will prompt the first super-administrative user on the domain to perform a one-time installation of a Client Name and API Scopes within the G Suite admin console's Manage API client access panel shown below. Why is this necessary? Learn more here.
The values needed for this step are provided in the Gopher for Gmail sidebar.