Hide thumbnail if there is a gallery on the post

Just use the public API functions: has_shortcode( $content, $tag );. It uses shortcode_exists( $shortcode ); internally to search the global $shortcode_tags array for the shortcode you are searching for. If that is successful, it uses get_shortcode_regex() to search for the actual shortcode – this will save you quite some time and avoid mistakes.

the_title();
if ( has_shortcode( get_the_content(), 'gallery' ) )
{
    the_content();
}
else 
{
    // show default view - example:
    the_post_thumbnail( 'your-desired-size' );
    the_content();
}

The gallery_shortcode( $post_id ); function might be of help as well.
This might not work if you are using a plugin that handles galleries different from how WP core does it. Anyway, make sure that you alter above has_shortcode() call to use the shortcode you are (or the plugin you are using) is using.