How to install codes of GTM both in the Head and in the Body?
Hi! You can add custom code in the header or body.
For header: Website → Website settings → SEO / META (tab) → Meta tags
For body: Asset manager → HTML - add your custom code after {{content}}
More info is here: https://help.sitejet.io/article/440-use-custom-html-css-and-javascript-code
Thank you!
While I had got the idea from help files that Meta tags should be used to add GTM Header codes but was not sure for Body tags.
Now I understand that GTM body tags should be added after the Content.
Beautiful and correct @Lucian_Dinu 
Will add that information to the article