The theme adds 2 additional attributes into WordPress native gallery shortcode:

• flexible – Set this to “1” for masonry style gallery layout. Setting of “0” (or leave it blank, or don’t set it at all) will display standard gallery layout (cropped images of same size).
• class – This CSS class attribute will be applied on each gallery item container HTML element. You can set custom classes on gallery items, such as frames, removing margins and so on (see theme user manual for more info on additional classes).

[gallery ids="1,2,3,..." flexible="0/1" class="" /]