Hi @jeronimo89. Let's use the terminology correctly.
Google Tag Manager - GTM
Google Analytics - GA
CSS - code for appearance
HTML - marking of your site content. It is in HTML to insert everything that you need.
GTM installed on the site once at the global level in template. Not to the single element like button. It creates in the HTML code of your site "windows", inside which you can use any code that insert in the GTM interface (in your GTM Google accaunt).
Based on the above, you must insert the GA inside the GTM on the side of your Google accaunt.
In order to add a target to the button, you need to configure it on the Google Analytics side using class of the button that you added to the addon so that the GA javascript is able to find this button in the code of your site and track the events.
GTM is optional. It is just a comfortable feature where you can add and manage many scripts from various services. If you just need to work with GA and nothing else, do not install GTM. Just add code from GA to your template. If you use Helix you enough to insert GA ID in the settings.