Gmail add-ons on iOS
Gmail add-ons have not yet been added to the iOS version of the Gmail app.
Change features in Gmail
The add-ons framework is designed to enhance Gmail—not to add limits. Consequently, you can't alter existing features or the Gmail user interface.
Use client-side scripting
Use Apps Script triggers
Charge users to install
We don't provide a way to charge users for installing add-ons, and add-ons can't include ads. However, you can roll your own payment system or call into an existing billing database. Your add-on can connect to non-Google services that bill users.