There are a few things you can't do with Gmail add-ons.
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.
SVG Image support
You can't currently use SVG images with
Card service widgets.
Have more than 100 widgets
For performance reasons, you can't add more than 100 widgets to a card section.