Options for an SEO gallery?

I believe your issue is that you’ve checked this box in the media settings:

Every image uploaded afterwards had it’s thumbnail cropped to fit those proportions. Unchecking the box will fix this for new uploaded images but your existing photos will still be cropped.

You can however regenerate the image thumbnails using the new settings. To do so you can use a plugin such as regen thumbnails. There are a few plugins that will do this, but they all do the job. Activate and regenerate your images and they should be proportional. The process may take a little time depending on the size and number of photos you’ve uploaded, but it only needs to be done once. Deactivate the regeneration plugin once you’ve finished