WordPress “get_post_class” get category as string

As the title states, I want to output the class of the category onto the looped queries.

At the moments I could use post class but I want to only add the one class and already have a set number of classes. For me this is all about getting 1 or more values from the array and echo’ing them out as a string into my loop template.

array(7) {
    [0]=> string(8) "post-170"
    [1]=> string(4) "post"
    [2]=> string(9) "type-post"
    [3]=> string(14) "status-publish"
    [4]=> string(15) "format-standard"
    [5]=> string(6) "hentry"
    [6]=> string(22) "category-uncategorised"
}

Currently

<?php $counter = 0;

$query = new WP_Query(array(
    'post_type'      => 'post',
    'posts_per_page' => '50'
));

if ( $query->have_posts() ) : while ( $query->have_posts() ) : $query->the_post(); ?>

    // Update Counter
    $counter++;

    <div class="newsfeed__blk <?php echo 'is--' . $counter; ?> col-md-4 col-sm-6 col-xs-12 <?php get_post_class([6]); ?>">...</div>

<?php endwhile; endif; wp_reset_query(); ?>

End Result

I want a newsfeed with testimonials. In wordpress I will assign a category to the post. When the page is rendered the query will add a custom class to differentiate the DOM element for my custom SCSS styles.