Customizing header section

Maybe try using the hooks?
http://demo.generatepress.com/hook-locations/
https://docs.generatepress.com/article/hooks-overview/

Ok i can try but how i add a widget elementor or plugin into a hook?

Hooks should accept everything - Shortcode, HTML, PHP.

yes i know that but i haven't a shortcode of countdown clock or elementor plugin

not sure what you mean? Can you copy and paste the content you are trying to add?

I need to add an offers bar with 3 sections: one of this section must contain a countdown clock and it have to be visible in all pages, is to say it should to be part of the layout header

In GP you can do this: https://docs.generatepress.com/article/split-header-three-sections/