Small grid gallery layout
The Small grid layout forces images to display in an fixed grid, like so:
and, with cropped images, possibly like so:

or with row height standardized, the grid could look like so:

The Small grid gallery layout settings can be found in the Admin Panel > Image gallery (any one) > Advanced > Advanced Layout Options > Small grid. These settings look like so:

Image size
This sets the displayed image size. Smallest or Thumbnail is recommended. Here's some more information on the available sizes.
Grid style
- Responds to screen size: grid adjusts to browser width (recommended)
- Fixed images per row: grid will remain fixed at this size
Standardize row height
This standardizes all image heights to the same as the shortest image in the gallery.