Force uploaded media to create a certain thumbnail size

thumbnail-function.php

// Add 250 x 250 thumbnail to every image uploaded
add_image_size( 'gallery-thumbnail', 250, 250, true );

// gallery_thumbnail is a custom size! It can have any name. 

// Call it later in a gallery loop.
        <section id="gallery_page">
          <ul class="gal columns-4">
            <?php 
$args = array(
        'post_type' => 'attachment',
        'post_mime_type' => 'image',
        'orderby' => 'post_date',
        'order' => 'desc',
        'posts_per_page' => '50',
        'post_status'    => 'inherit',
        'category_name' => 'Gallery Page'
         );

     $loop = new WP_Query( $args );

while ( $loop->have_posts() ) : $loop->the_post();
$thumb = wp_get_attachment_image_src( get_post_thumbnail_id($post->ID), 'gallery-thumbnail' );
$url = $thumb['0']; 
$image = wp_get_attachment_image_src( get_the_ID() ); 
echo "<li><img src='" . $thumb[0] . "'></li>";

endwhile; ?>

          </ul>
        </section>