If you have already authenticated with Google but want to download a google spreadsheet from a different Google Account, follow these steps:
Choose "Reauthenticate With Google" in the Window→Game Data Editor→Import Spreadsheet Menu.
Choose "Go to Google Authenticate URL." The google authenticate prompt should open in your browser.
Grant Access to Game Data Editor. The google authenticate will ask you to grant ReadOnly access to your google spreadsheets. Click Accept to continue the authentication process.
Copy the access code from your browser and paste it into Set GDE Access Code window.
Select the Spreadsheet to Import and click "Download"
Done! Your data will now appear in GDE Editor and you will be able to read your data during runtime.