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' );