Actually, WordPress does have a nice gallery shortcode, which will let you specify a post ID from which it will grab attachments.
Implementing that in your template is just a matter of...
<?php echo do_shortcode('[gallery id="some_number"]'); ?>
There are a whole host of other options, so you should check that out.
If you like to write code others have already written, you can roll your own gallery function.
function wpse31222_gallery( $post_id )
$attachments = get_posts(
'post_type' => 'attachment',
'post_mime_type' => 'image',
'numberposts' => -1,
'post_parent' => $post_id
foreach( $attachments as $a )
* do stuff here
* functions like wp_get_attachment_image_src will come in handy
Then use it somewhere in your template...
<?php wpse31222_gallery( $some_post_id ); ?>
From your question, it seems like you're looking for some sort of system that you could use to give your users the option of managing a gallery's.
So, what I'd suggest is you register a post type, and only give it limited functionality (a title). You could then give users either a custom meta box or provide some way for them to easily get a shortcode to paste into any page or post. I did this recently on a plugin by adding a little "action link" to the posts screen (the list table of posts) and using smoke.js to overlay an alert with the shortcode for the user to copy.
Grabbing images associated with posts is the easiest way. You're not limited to accessing the current post, fortunately.