A gallery default setting is to show all the images attached to a given post. So, to display a gallery of images in one post, and then an additional image not included in the gallery, you have two solutions:
1. Attach all the gallery images to your post, and leave the one image you don't want to include in the gallery unattached (you may do so by uploading it from the Media > Add New section of your dashboard instead than uploading it form the Post Edit page ).
2 Attache all the images to your post, then use the "exclude" by image ID number parameter of the gallery shortcode to exclude the image that you don't want into the gallery.
More information about the gallery shortcode here > http://en.support.wordpress.com/images/gallery/
An excellent refernce for working with galleries here > http://gammagirl.wordpress.com/2008/07/27/not-the-official-wordpresscom-gallery-faq/