WordPress 3.9 ships with the ability to add HTML5 support for galleries which will now use figure and figcaption to display images. This is a huge change for the better! Bonus: br tag used for clearing rows has been cleaned out as well. While all of this is good news for more semantic markup, one might now be left wondering how to style the new …

gallery-style.css

/* WordPress Gallery 
------------------------------------------ */

.gallery {
	margin-left: -0.5em;
	margin-right: -0.5em;
}

.gallery:after {
	display: table;
	content: "";
	clear: both;
}

.gallery-item {
	margin: 0;
	float: left;
	padding: 0.5em;
}

.gallery-item img {
	width: 100%;
}

.gallery-columns-1 .gallery-item {
    width: 100%;
    float: none;
}
.gallery-columns-2 .gallery-item {
    width: 50%
}
.gallery-columns-3 .gallery-item {
    width: 33.3333%
}
.gallery-columns-4 .gallery-item {
    width: 25%
}
.gallery-columns-5 .gallery-item {
    width: 20%
}
.gallery-columns-6 .gallery-item {
    width: 16.6667%
}
.gallery-columns-7 .gallery-item {
    width: 14.2857%
}
.gallery-columns-8 .gallery-item {
    width: 12.5%
}
.gallery-columns-9 .gallery-item {
    width: 11.1111%
}

.gallery-columns-2 .gallery-item:nth-child(2n+1),
.gallery-columns-3 .gallery-item:nth-child(3n+1),
.gallery-columns-4 .gallery-item:nth-child(4n+1),
.gallery-columns-5 .gallery-item:nth-child(5n+1),
.gallery-columns-6 .gallery-item:nth-child(6n+1),
.gallery-columns-7 .gallery-item:nth-child(7n+1),
.gallery-columns-8 .gallery-item:nth-child(8n+1),
.gallery-columns-9 .gallery-item:nth-child(9n+1) {
	clear: left;
}

The guys from Automattic use a different approach with inline-block instead of floats (https://github.com/Automattic/_s/blob/master/style.css#L640-L676).

Genesis users could also use the built Bootstrap column classes couldn't you?

Here's another way to fix the problem http://www.studiopress.com/forums/topic/executive-pro-wordpress-native-gallery/#post-126397