The plugin currently allows you search the title, caption and content by enabling it on the settings page. It works with the standar website search and it will display all other content.

If you want to return only galleries all you will have to do is add a standard seach forma add a hidden field with the content type

<input type="hidden" name="post_type" value="ims_gallery" />

http://wordpress.org/support/topic/limit-search-results-to-custom-post-type