I created an entirely fresh install on GP on a new staging site and started from scratch. I created my Elementor header and set it to appear on all Posts. When I go to create my Post template in Elementor, the GP nav bar was still there, so I went and disable the nav like you said and ticked no tagline and no site title.
However, there is still a blank element showing up at the top of the page that doesn’t appear to be from Elementor. Any ideas how to remove it? This is with the canvas set to full width and as you can see my Elementor header doesn’t show either. Screenshot https://i.imgur.com/WSqtzHU.png