This is very nearly a duplicate question but I can't figure out where to insert .all() to execute my element query. I'm still asking because I may be messing up php, function or filter syntax somewhere. I've looked through other related answers (1, 2).

I traced the error to the template(s) below. The first _entry.twig chooses a layout based on url parameters and Admin CP settings:

{% block content %}
  {% if craft.app.request.segments|length == 3 %}
    {% set entry = craft.entries().slug(slug|literal).one() %}
    {% include "shared/_grid-item" with {
      entry: entry,
      index: number,
    } only %}

  {% elseif entry.galleryShowThumbnailPage|length %}
    {% include "shared/_grid" with { entry: entry } %}

  {% else %}
    {% include "shared/_gallery" with { entry: entry } %}
  {% endif %}
{% endblock %}

I'm fairly sure the error is not coming from _entry.twig but may be caused by the variable entry in the shared/_grid-item.twig partial it gets passed to:

{% set images = entry.galleryImages.withTransforms(['sideScrollImage', 'tinyThumb']).all() %}
{% set itemImage = images[index - 1] %}
{% set itemImages = []|merge([itemImage]) %}
{% if itemImage.relatedImages.exists() %}
  {% set itemImages = itemImages|merge(itemImage.relatedImages) %}
{% endif %}

{% set next = entry.url ~ '/' ~ (index + 1) %}
{% set prev = entry.url ~ '/' ~ (index - 1) %}
{% if index == 1 %}{% set prev = false %}{% endif %}
{% if index == (images|length) %}{% set next = false %}{% endif %}

{% extends "_layouts/_default.twig" %}

{% block content %}
    <main role="main" class="content">
      <h1 class="content__title content__title--hidden">
        {{ entry.title }}
      </h1>

      {% include "_partials/_sideScroll" with {
        back: entry.title,
        images: itemImages,
        next: next,
        prev: prev,
      } only %}
    </main>
  {% cache %}
  {% endcache %}
{% endblock %}

My understanding of the element query is that I may be call a for loop somewhere but not executing it by calling .all(). I'm not sure if any of the code in _grid-item.twig would call a for loop I'm unaware of – maybe in {% set itemImages = []|merge([itemImage]) %} or {% set itemImages = itemImages|merge(itemImage.relatedImages) %}? Or if some of the php syntax needs to change to allow adding .all()?

I've tried adding .all() various places like {% set itemImage = images.all()[index - 1] %} but everything throws a twig runtime error.