How to use HtmlService in Gmail add-on using App Script

Question

I am creating a Gmail Add-on. The following reference page says - https://developers.google.com/gmail/add-ons/reference/

\"Gmail add-ons are built using Apps Script

Answer 1

TL;DR:

To build interfaces for Gmail add-ons, you must use the Card service instead [of the HTML Service].

Quoted from your reference, under HTML service.

For popups, +1 to @akshay who recommended OVERLAY: CardService.newOpenLink().setOpenAs(CardService.OpenAs.OVERLAY), which will "Open as an overlay such as a pop-up". See CardService OpenAs.