GeneratePress Premium already allows its users to create custom headers, heroes & footers, and this functionality can be further extended with a Navigation Menu Block.
With a navigation menu block, a user will have complete control over the site design and won’t have to rely on the theme’s navigation menu customization.
Otherwise speaking, it would be something like Beaver Themer that allows the users to create fully customizable headers & footers with navigation menus.
A menu/navigation block is included with the gutenburg plugin. I have used this with GB block hook successfully for adding a custom menu to the header in addition to the standard menu.
These are experimental features and good to try them out on the sandbox site. But otherwise and by all means, I would avoid them on a live website.
Do I have to use the Gutenberg plugin to get access to these features?
It depends on the feature you want to use! Keep in mind that each version of WordPress after 5.0 comes with bundled versions of the Gutenberg plugin, automatically containing new features and changes. If you want the cutting edge features, including more experimental items, you will need to use the plugin. You can read more here about whether using the plugin is right for you.
Besides, the level of customizations, features & options that GenerateBlocks would offer can’t be expected in the default Gutenberg blocks.