category.php to be changed

Hi Tom,
I'm using the PlugIn wp-masonry-layout-pro in posts and I'm suposed to change the category.php which doesn't exist apparently in GeneratePress.
The develeoper of the PlugIn asked me kindly to ask you where to change the code.
("Hi,
It better if you can ask the theme support and find out where you should edit the file to change category layout.
If there are unable to do so, i can help you. But it will take time for me. As they are developer of this and this can quickly point it out for you.
Thanks")
The following code, supposed to be found in category.php

<?php /* Start the Loop */ ?>
            <?php while ( have_posts() ) : the_post(); ?>

                <?php
                    /* Include the Post-Format-specific template for the content.
                     * If you want to override this in a child theme, then include a file
                     * called content-___.php (where ___ is the Post Format name) and that will be used instead.
                     */
                    get_template_part( 'content', get_post_format() );
                ?>

            <?php endwhile; ?>

            <?php generate_content_nav( 'nav-below' ); ?>

which exists only in the index.php of the parent theme is supposed to be changed into

<?php echo do_shortcode('[wmls name="name of shortcode" id="n°"]') ?>

but this doesn't help within the index.php...
can you please direct me to the correct file to be changed?
(as you can see I don't know anything about php).

the point is to keep the layout of http://goo.gl/jLURCj when filtering by categories (image only with text on hover) for example "sammlung ***" as an example category (= "kategorien" in german).

best
Torsten