How to set default category for custom post type ?

I set custom post type and i added taxonomy (category) to in it. But i couldn’t add default taxonomy (category). I have searched about 2 or 3 hours but i have nothing.

Let me describe more. For example, when you write something in post and published, if you didn’t set category, WP assign “Uncategorized” category automatically. I want it to assign a default taxanomy for my custom post type like that.

Here is my code :

register_taxonomy('gallery_cat','gallery',array(
		'hierarchical' => true,
		'label' => __('Gallery Category','yacht'),
		'query_var' => true
	));

I am waiting your helps. Thanks.