Force all thumbnails to be square for a uniform appearance.
Thumbs are created square to the center of the image.
You must upgrade your G3 install to the latest version as of this writing. Verify by going to your modules page and see if Gallery core application is 41 or greater.
This module may be downloaded from the Gallery-contrib git repository here
Click the download button for the module you want. Extract the square_thumbs folder to your gallery3/modules directory.
Activate the module: Login -> Admin -> Modules. Check the box next to the module.
Click update at the bottom of the page.
Similar module: Gallery3:Modules:rectangle_thumbs