Gallery blocks have two settings: the number of columns, and whether or not images should be cropped. The default number of columns is three, and the maximum number of columns is eight.
Below is a three column gallery at full width, with cropped images.
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Sunburst over the Clinch River, Southwest Virginia. 
Orange Iris 
Boardwalk at Westport, WA 
Bell on wharf in San Francisco 
Coastline in Huatulco, Oaxaca, Mexico
Some more text for taking up space.
A two column gallery, aligned to the left, linked to media file.
In the editor, the image captions can be edited directly by clicking on the text.
If the number of images cannot be divided into the number of columns you have selected, the default is to have the last image(s) automatically stretch to the width of your gallery.
A four column gallery with a wide width:
A five column gallery with normal images:
This is the same gallery, but with cropped images.
Six columns: does it work at all window sizes?
Boardwalk at Westport, WA Golden Gate Bridge Sydney Harbor Bridge Bell on wharf in San Francisco Rusty rails with fishplate, Kojonup 
Boats and reflections, Royal Perth Yacht Club 
Antique farm machinery, Mount Barker Museum, Western Australia 
Raindrop ripples on a pond 
Albany wind-farm against the sunset, Western Australia 
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shrouded in fog at a farm outside of Walker, Iowa 
Jericoacoara Ceara Brasil 
Sunrise over the coast in Huatulco, Oaxaca, Mexico 
Seven columns: how does this look on a narrow window?
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Eight columns:
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