Hi Michiel,

Try to add your post type name in the file:

/wp-content/plugins/thegem-elements/inc/shortcodes/shortcodes.php

Line 2100

'blog_post_types' => $post_types ? explode(',', $post_types) : array('post', 'thegem_news'),

and modify 

function thegem_vc_get_blog_categories() 

line 22152 

you can see example here:

if(taxonomy_exists('thegem_news_sets')) {
        $terms = get_terms('thegem_news_sets', array('hide_empty' => false));
        foreach ((array)$terms as $term) {
            $categories[$term->name.' ('.__('News', 'thegem').')'] = $term->slug;
        }
    }