I recently had to rebuild my galleries page due to a problem with a Bluehost update.
Previously, my Auto Index images on the galleries page (parent) were sized as 16 x 9 /2 columns and were static when the the page was resized. http://davidbuddphotography.com/galleries.php
Now when the page is resized the columns change to three. However, the "child" images still retain the 2 column format when that page size is changed. http://davidbuddphotography.com/galleries/01-places/
The galleries were rebuilt via CE4 Publisher using the original Auto Index templates from the previous galleries
they should behave the same then. Looking at the Galleries page source code, it's designed to be three columns. Try exporting Pages again and uploading. You can probably get away with uploading the galleries.php file but you may need to upload the lib/ folder too.