Yes, I did. You are absolutely right in all of your comments. And I am very grateful for your attention and time given to me. The thing is that I did not immediately understand where and how to override the file page_header.html.twig which was actually in the gantry 5 plugin folder. As a result, I did everything but with switch. I am very grateful to you, mark your decision as correct as only thanks to you I managed to understand, well, I wrote my decision on the basis of yours. Thanks again.
PS. Just want to note that the solution for describing the categories that is indicated here:
I did not work, although I tried all possible options.