GTM container in block appears as regular text even though set to be Full-HTML
I've discovered that CKeditor made the code &lt;p&gt; so I tried to replace the &lt;p&gt; tag of the input to  but it didn't help, so as alternative I tried to format the input as with PHP text-foramt, But this also didn't help.

Working solutions would be:


Putting the code somewhere (at the first row for example) of you page.tpl.php.
Installing the module.


If you plan to change your sites theme and going to change some themes in the near future, better pick option 2 --- It will save you from copying the code time and again to each new page.tpl.php.