You'll need to enable Google Login to connect Google Sheets. Docs.
Navigate to https://console.developers.google.com/apis/library and select your Retool project.
Search for the following three services, in the Library dashboard, and enable all of them:
Google Drive API
Google Sheets API
Navigate to your settings page in Retool, and then press Add resource.
Select Google Sheets in the dropdown of integrations. Give it a name, and then press Authorize Google Sheets.
Follow our Google Sheets tutorial.