but what it should be is an extension app usable on any browser, especially on your phone.

It was that, and it was blocked by Google (app store), Apple (app store), Firefox, Apple (Safari), and Google (Chrome).

The only way to install it was to put your browser in developer mode and load it in yourself, which most people didn't bother with, so the Gab team decided to just release an entire browser instead, which should have been easier to use than the extension.