Hi Rik,
I looked at your repo. I’ve no experience with Netlify but I could see your site with hugo server. Good, that you’re working with Tachyons.
If you want to see which template is used just make a temporary remark in all templates like <h1>This is template gallery.html</>. If you want to dedicate a specific template to a page you can use Front Matter. I described this in my post My experiences with Hugo's template lookup order in the different layout variants.
Hope this helps.