The Khan2GC add-on can be installed from the Google Workspace Marketplace. The link below will take you to the installation page.
To start using Khan2GC, in Google Drive, create a folder that will be solely for your Khan2GC spreadsheet. Inside the folder, create a new spreadsheet. The title of the spreadsheet and folder do not matter.
After creating the new spreadsheet, open it, and find the Extensions menu on top. Select Extensions > Khan2GC > Enable. This gives Khan2GC permission to operate in the spreadsheet going forward.
Navigate to Extensions > Khan2GC > Setup > Launch Setup
The setup sidebar will guide you through the setup process for Khan2GC. Each step should be completed and in order to ensure the spreadsheet is setup correctly.