https://www.advancedcustomfields.com/resources/group/
$images = get_field('{GROUP_FIELD_NAME}_gallery');
$group = get_field('{GROUP_FIELD_NAME}');
$images = $group['gallery'];
if (have_rows('{GROUP_FIELD_NAME}')) {
while (have_rows('{GROUP_FIELD_NAME}')) {
$images = get_sub_field('gallery');
}
}