Without knowing exactly what you’re trying to acheive, it’s difficult to know if using page templates for this is the best solution, or if you should consider using a Custom Post Type for portfolio items, and then a custom archive page to display them.
Anyway, I digress…
The template a page is set to use, is defined in the post meta using the key _wp_page_template. If you use the query_posts method instead of get_pages you can also use the meta values as part of the query.
In my example, the filename of the Portfolio page template is page-portfolio.php. You’ll need to change the query below to match yours.
query_posts(array(
'post_type' =>'page',
'meta_key' =>'_wp_page_template',
'meta_value'=>'page-portfolio.php',
));
If you use this query in your template file, you can then use the normal WordPress loop to iterate over the results. You can also add all the other possible parameters to the query to refine it further if you wish.