G Suite Marketplace
https://gsuite.google.com/marketplace/category/works-with-gmail
As an add on the right bar.
Examples:
https://gsuite.google.com/u/0/marketplace/app/evernote_for_gmail/294974410262
https://gsuite.google.com/u/0/marketplace/app/trello_for_gmail/710372432947
Archbee supports Googel Docs/Sheets/Slides