Put whatever you like to show in the .marker div
Something like
<div class="marker" data-lat="<?php echo esc_attr($location['lat']); ?>" data-lng="<?php echo esc_attr($location['lng']); ?>">
<strong><?php echo esc_attr($gm_headline); ?></strong>
<p><?php echo esc_attr($gm_description); ?></p>
</div>
