G Suite Updates Blog: G Suite Add-ons now generally available in Calendar, Gmail, and Google Drive

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Last year we announced the beta of G Suite Add-ons, a new cross-suite platform that connects G Suite to your favorite workplace apps. Beginning today, G Suite Add-ons will begin rolling out to all users.

Whilst the Add-on ecosystem is being extended making them generally available to end users developers will have to wait until you can start developing new add-ons for G Suite:

Organizations can also build their own add-ons using Apps Script. Note that the developer feature will be fully available in early February

Source: G Suite Updates Blog: G Suite Add-ons now generally available in Calendar, Gmail, and Google Drive