You cannot mix get_sub_field() and have_rows().
$group_parent = get_sub_field('group_parent');
gets an array, including the repeater sub field and its sub fields.
Once you do this then you cannot use have_rows() on the sub fields of this field because you are already inside a have_rows() loop for ‘group_parent’
basically you either need to loop over the array returned without using have_rows() or you always need to use have_rows(). You cannot loop over a sub fields that is a repeater or a group without looping over the parent repeater or group.
// have rows
while( have_rows('parent_gallery') ) ....
while (have_rows('group_parent')) .....
while( have_rows('children_galleries') ) .....
// using the array, I'm not real sure about this but hopefully you get the idea
while( have_rows('parent_gallery') ) : the_row();
$group_parent = get_sub_field('group_parent');
....
$children_galleries = $group_parent['children_galleries'];
foreach ($children_galleries as $gallery)
at any rate, if you output the array you’ll see what you’re working with
$group_parent = get_sub_field('group_parent');
echo '<pre>'; print_r($group_parent); echo '</pre>';
at each nested level you have a choice, get the sub fields contents or use another have_rows() loop. If you get the sub field values then from that point on you must use the array that is returned and you can no longer use have_rows() for any of its sub fields that are repeaters or groups.
