Hi,
Thanks for contacting us. A good approach would be to use a Custom Header layout in Helix Ultimate rather than the predefined header. You can create separate rows for the menu and logo, center both, and then use Custom CSS to make the header transparent/positioned over the homepage slideshow or inner-page header images. Helix Ultimate supports custom header layouts, but the transparent overlay effect currently requires custom CSS.
For the menu centering, Helix also documents using justify-content: center on the menu wrapper.
So yes, this design should be possible with Helix Ultimate. You may need some custom CSS to reproduce the Gantry site's header styling exactly.
-Regards.