Once you have BG Database installed, Open the BG Database window (Window -> BG Database) and then install the Google Sheets plugin:
(You can just click 'Import Silently' or if you're paranoid click the interactive import to see the normal .unitypackage import dialog).
Setup Google Cloud project
This IS a requirement for you to be able to read from Google Sheets (basically you just need to enable the Google Sheets API in Google Cloud and add a credential).