Is Merchant Mail available on iPhone or iPad?
Gmail Add-ons are not yet available on the mobile version of Gmail for iOS. This means that this add-on cannot be used on the Gmail app on iOS devices (iPhone and iPad). Google has not yet announced a release date for add-ons on iOS mobile, but it should be coming soon. As a workaround, the add-on can be used on iOS if you access Gmail through your web browser (ie. Safari or Chrome).
The add-on can currently be used on the web version of Gmail (using a browser) or on the native apps on Android devices.