I tried plugging in the for loop and it seemed to stop working. It looked like this:
//Fetch the gallery for the selected post
$gallery = get_field('gallery', $post_id);
if($gallery){
//loop through the images in the gallery
for($i = 0; $i > count($gallery); $i++){
$gallery[$i]['constrain'] = get_field('constrain', $gallery[$i]['id']);
}
}
I have a working model, but since you suggested it I wanted to try it.