How to create a gallery
You can create a gallery in The Grid editor by uploading 9+ images in one post. This will create a gallery of the images in that post. You can do a redesign to change the outcome, for example, to display your images in circles, columns, etc.
In order to create a gallery, the post has to be set to be on your Homepage. If it's only linked to Nav, and not set to show on your homepage, a gallery will not be created. See this article about post settings.
If this is a deal breaker for you here's what you can do to both link to Nav and have a gallery:
Use one of your other sites to create a Homepage that hosts your images, either in one post, as seen above, or in several posts. When you do this, it will style the page for you as a gallery, or as images in multiple posts. You can then Link to External Website in Nav on your main site, and link to your image gallery site.