Gopher for Gmail can be installed by anyone via the Chrome Web Store, but to access its sidebar you must be a G Suite domain super administrator.
Per-user, one-time installation step:
Each user must install Gopher for Gmail from the Add-ons gallery from any Google Sheet, or use this link, and click "Install." After installation, the tool becomes available across all of the user's Google Sheets, via the Add-ons menu.
Per-domain, one-time installation step:
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.