I was asked via chat how to do show 3 columns in the gallery instead of 4, so here are the steps:

1. Change gallery.liquid

Open gallery.liquid (Theme > Template Editor in the Snippets section).

You'll see an if test at the top. Change it to:

{% if true or template == 'collection.sidebar' %}

This changes it to always assume a 3-col layout.

Now, change the image size from 'compact' to 'medium':

<img src="{{ product.images.first | product_img_url: 'medium' }}" alt="{{ product.title | strip_html | escape }}"/>

2. CSS Changes

Open styles.css.liquid and at the bottom add the following CSS (this will override the existing rules defined further up in the file):

/* 3 col gallery */
#products .product {padding:0 15px; width:300px;}
#products .image {height:288px; width:288px;}
#products .first_col {padding-left: 0;}
#products .last_col {padding-right: 0;}

That's it!