Post gallery loop with thumb and full size

There might be other possibilities to this, mine is given below ..

1. First you use the get_post_gallery function to store the image id’s in a separate array ..

   if ( get_post_gallery() ) {
   
       $gallery        = get_post_gallery( get_the_ID(), false );
       $galleryIDS[]   = array();
   
       /* Loop through all the image and store them one by one */
       foreach( $gallery['ids'] as $ids ) {
   
           $galleryIDS[] = $ids;
   
       }
   }
   

2. Then you output each image by id using wp_get_attachment_image as shown below ..

   <?php foreach ($galleryIDS as $key => $value ) { ?>
   
       <div class="galerija-box">
               <a href="https://wordpress.stackexchange.com/questions/213601/<?php echo wp_get_attachment_image( $value,"medium' ) ?>"  rel="lightbox">
                   <img src="https://wordpress.stackexchange.com/questions/213601/<?php echo wp_get_attachment_image( $value,"full' ) ?>"/>
               </a>
       </div>
   <?php } ?>
   

I didn’t test the code so it might have some syntax issues, but the method should work for you ..

EDIT

As mentioned before I didn’t check the code if its functional or not. So I tested it out, found a few bugs, fixed them. Below is the working code ..

if ( get_post_gallery() ) {

    $gallery        = get_post_gallery( get_the_ID(), false );
    $galleryIDS     = $gallery['ids'];
    $pieces         = explode(",", $galleryIDS);

    foreach ($pieces as $key => $value ) { 

        $image_medium   = wp_get_attachment_image_src( $value, 'medium'); 
        $image_full     = wp_get_attachment_image_src( $value, 'full'); 
    ?>


    <div class="galerija-box">
            <a href="https://wordpress.stackexchange.com/questions/213601/<?php echo $image_medium[0] ?>"  rel="lightbox">
                <img src="<?php echo $image_full[0] ?>"/>
            </a>
    </div>
    <?php 
    }
}
?>

Reference: https://codex.wordpress.org/Function_Reference/wp_get_attachment_image_src