I would create a Top site that just has the seven top pages and a NavBar for these.
I would then create a second Gallery site for the six gallery pages.
The six images on the Top site gallery page link into the Gallery site pages. You have to use Link to Web addressing like gallery-3.htm, where the extension is required as the Top site doesn't know about the Gallery site.

The Gallery site has its own NavBar.
You can use the same NavBar template for a secondary NavBar with one button called Gallery that returns to the Top site gallery page.

Do the same for the four pages in a separate About site.

All three sites can publish to the same folder provides all page names are unique across all of them.
You can introduce sub-folders but I have never found much benefit in doing so.

Please note the first page of a site does not need to be called index so the Gallery and About first pages can be anything like gallery-1 and about-1.

All three sites can grow and change NavBars with impunity.

Acorn