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. 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.
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Sunburst over the Clinch River, Southwest Virginia. 
Orange Iris 
Boardwalk at Westport, WA 
Bell on wharf in San Francisco 
Sunrise over the coast in Huatulco, Oaxaca, Mexico 
Some more text for taking up space.