Modify WP theme's gallery behavior

Sorry if my question isn’t written in the right category or somehting. I haven’t found any topic about this. I’m not a developper and I am new to WordPress so my question might seems a little naive.

I bougth a WP template at ThemeForest that has a gallery function/template. I wish to modify it’s behavior so that, instead of showing images from a category by clicling on the button related to it, the images from a particular category are shown automatically when the page loads. Currently, the images shown when the page is loaded seems to be selected randomly among all the existing categories. I want to get rid of the selecting option with the buttons and just show the images from a single category on a single gallery page. In fact, I would need three independent gallery pages each showing it’s own category images. Actually, I need to simplify the gallery jQuery function I guess. I thought I might need to duplicate the _gallery.php template so that I could have three different galleries but I am not sure. Here is the gallery template code that generates the gallery and below is a link to the theme I bougth:

<!-- GALLERY -->
        <div class="wrapper">

                            <?php
                              global $wpdb;
                              $post_type_ids = $wpdb->get_col("SELECT ID FROM $wpdb->posts WHERE post_type = 'pt_gallery' AND post_status = 'publish'");
                              if(!empty ($post_type_ids )){
                                $post_type_cats = wp_get_object_terms( $post_type_ids, 'category',array('fields' => 'ids') );
                                if($post_type_cats){
                                  $post_type_cats = array_unique($post_type_cats);
                                  $allcat = implode(',',$post_type_cats);
                                }
                              }
                              $include_category = null;
                            ?>

             <div class="speakers-filter left">
                    <h3>Quelques photos de l'&eacute;v&eacute;nement :</h3>

                <div class="page-portfolio-categories left">
                    <div class="bg-portfolio-categories left">
                        <div class="portfolio-categories left">
                            <ul>

                                <div class="color-buttons left">
                                    <li class="paragraphp cat_cell_active cat_cell" rev="<?php echo $allcat?>">
                                        <div class="small-black">
                                            <div class="small-black-center cat_cell_active left cat_cell"><a href="#"><?php _e('Tout', 'eventor')?></a></div>
                                        </div>
                                    </li>
                                </div>

                              <?php
                            if(!empty ($post_type_ids )){
                                 foreach ($post_type_cats as $category_list) {
                                    $cat = 	$category_list.",";
                                    $include_category = $include_category.$cat;
                                    $cat_name = get_cat_name($category_list)
                                ?>

                                <div class="color-buttons left">
                                    <li rev="<?php echo $category_list?>" class="cat_cell">
                                        <div class="small-black">
                                            <div class="small-black-center left"><a href="#"><?php echo $cat_name?></a></div>
                                        </div>
                                    </li>
                                </div>

                                <?php } }?>
                            </ul>
                        </div><!--/portfolio-categories-->
                    </div><!--/bg-portfolio-categories-->
                </div><!--/page-portfolio-categories-->

            </div><!-- /speakers-filter -->

                                <div class="portfolio-loader" id="portfolio-loader" style="position: relative;top: 300px;"></div>

            <div class="portfolio-content left">

                        <script type="text/javascript">
                            jQuery(document).ready(function(){
                                jQuery('.cat_cell_active').click();
                            });
                        </script>

                        <script type="text/javascript">
                            jQuery('.cat_cell').live('click',
                            function () {
                                var id = jQuery(this).attr('rev');
                                jQuery('.cat_cell').removeClass('cat_cell_active');
                                jQuery(this).addClass('cat_cell_active');
                                jQuery('.ajax_holder').animate({opacity:0},500,function(){
                                    jQuery('.portfolio-loader').show().animate({opacity:0},0).animate({opacity:1},500,function(){
                                        var randomnumber=Math.floor(Math.random()*100000000);
                                        var postAjaxURL = "<?php echo get_template_directory_uri() ?>/_galleryajax.php?id="+id;

                                        jQuery('.ajax_holder').load(postAjaxURL, {rand: randomnumber},function(){
                                            jQuery('.portfolio-loader').animate({opacity:0},500).hide();
                                        });
                                    });
                                });
                                return false;
                            });
                        </script>

				<div class="ajax_holder"></div><!--AJAX Holder-->

                 <?php

The theme. Just take a look at the gallery section:

http://themeforest.net/item/eventor-event-management-wordpress-theme/full_screen_preview/2437242

Thanks in advance for helping!!!