How to get the next and previous image title from attachment or gallery?

You are using an associative array of $attach_id => $attach_obj and loop through it. While there are methods to get the next and previous elements in this array, I would prefer to rely on continuous index array to get these objects. First convert the array:

$images = array_values($images);

Now loop through using the contineous index:

foreach ($images as $index => $attachment) {

Finally check for next and previous objects:

<?php if (isset($images[$index+1])) { ?>
    , "next": "<?php echo $images[$index+1]->post_title; ?>"
<?php } ?>
<?php if (isset($images[$index-1])) { ?>
    , "prev": "<?php echo $images[$index-1]->post_title; ?>"
<?php } ?>

Edit: here how your code might look like:

<script>
    var clist_autoplay = false;
    var ctemp_autoplay = clist_autoplay;
    jQuery(document).ready(function($) {        
        var larr = {
<?php
$num = 1;
$images = array_values($images);
foreach ($images as $key => $attachment) {
    $image_img = wp_get_attachment_image_src( $attachment->ID, 'large' );
    $alt = get_post_meta($attachment->ID, '_wp_attachment_image_alt', true);
    $image_title = $attachment->post_title;
    $caption = $attachment->post_excerpt;
    $description = $attachment->post_content;     
?>
            "<?php echo $image_title ?>": {
                "0": "<?php echo  $image_img[0] ?>",
                "1": 600,
                "2": 400,
                "3": "<?php echo $image_title ?>",
                "4": "#",
                "5": "<?php echo $image_title ?>",
                "6": "",
                "7": "",
                "8": "",
                "9": "",
                "num": <?php  echo $num++; ?>,
                "slideTitle": "<?php echo $caption ?>",
                "slideContent": "<p><?php echo $description; ?></p>",
                "slideNum": null
                <?php if (isset($images[$key+1])) { ?>
                    , "next": "<?php echo $images[$key+1]->post_title; ?>"
                <?php } ?>
                <?php if (isset($images[$key-1])) { ?>
                    , "prev": "<?php echo $images[$key-1]->post_title; ?>"
                <?php } ?>
                }
<?php } // end foreach ?>
    }
}