HI!

pls. use following css:

li.menu-item.menu-item-search {
    display: none !important;
}

or in the TheGem child theme add next code to functions.php file:

function thegem_child_init() {
    remove_filter('wp_nav_menu_items', 'thegem_menu_item_search', 10);
}
add_action('init', 'thegem_child_init');

Pls. give link to your site.

Css:

.vertical-minisearch {
    display: none !important;
}