You can switch the layout of your gallery thumbnail grids through the Theme Editor.
- While viewing your selected theme within the Theme Editor, click on Customize Image Grids.
- In the Layout section choose from Follow Gallery Settings, Masonry (Vertical) and Subway (Horizontal).
- Once you've selected your preferred layout, click Save at the top.
Please Note: This will change the setting for all of the galleries that are associated with the theme that is being edited. If you'd like to change the setting for a set of specific galleries instead, click the Save As option at the top and save your changes to a new theme. Once the new theme has been saved you can head back to your galleries (or gallery types) and select the newly created theme.
For more information on Theme's please visit this page.