There are several ways to do this. Below outlines one approach.
Rather than use the image gallery feature, which is really just intended to display gallery images, perhaps what you want is to display rows of smaller images, each of which links to another web page (internal or external). For example, check out this effect at: https://demo.getpublii.eu/themes/persona/ — in which they use a maximum of 4 initial smaller images (or two on mobile devices), and each image links to another page.
Maybe you could create your version of this for your site. If yes, you would need some HTML code for the page that you want to display your grid of smaller, thumbnail images, plus some additional CSS code to add into the Publii: Tools & Plugins > Custom CSS page.
If you’re not sure how to do it, one option might be to contact the GetPublii folks at: https://getpublii.com/customization-service/