What is #gkHeader ?

Good day! Prompt, where the class # gkHeader. What it is and for what?
The forum topic is ignored - https://www.gavick.com/support/forums/119/12029.html. This theme - another little attempt to resolve the pressing issue.

It is one of the HTML elements ID, you can find it file layouts/blocks/header.php in most of our templates.

I can change the tag <h1> to <div>, and the tags <h2-6> to tags <h1-5>?

Yes but you also need to change proper styles in CSS files, I advise you to use firebug to check what files you need to modify.

You may suggest the names of css?