Default Category Page not showing custom post type which has taxonomy category

The default query doesn’t include custom post types. You have to use pre_get_posts hook to modify your query. Hope this solves your issue

function custom_posttype_query( $query ) {
    if( (is_category() || is_tag()) && $query->is_archive() && empty( 
        $query->query_vars['suppress_filters'] ) ) {
        $query->set( 'post_type', array(
           'post', 'gallery'
        ));
    }
    return $query;
}
add_filter( 'pre_get_posts', 'custom_posttype_query' );