This page states:
The contextual cards you create for your add-ons work for both web and mobile versions of Gmail. This means that you don't need to create separate web and mobile versions of the add-on — the same code works everywhere!
Note: Gmail add-ons have not yet been added to the iOS version of the Gmail app.
However, if I browse https://mail.google.com from Chrome for Android browser (on mobile phone), I don't see the cards in the mobile website of Gmail, and add-on actions don't seem to work.
Is there a trick to enable it?