Ansible - using with_items and when conditional to

You do not need to register your result with the item salt. When you register the result of a loop (e.g. with_items) the registered value will contain a key results which holds a list of all results of the loop. (See docs)

Instead of looping over your original device list, you can loop over the registered results of the first task then:

- name: Check if the disk is partitioned and also ignore sda
  stat: path=/dev/{{item}}1
  with_items: disk_var
  when: item != 'sda'
  register: device_stat

- name: Create GPT partition table
  command: /sbin/parted -s /dev/{{ item.item }} mklabel gpt
  with_items: "{{ device_stat.results }}"
  when:
    - not item | skipped
    - item.stat.exists == false

The condition not item | skipped takes care of that elements which have been filtered in the original loop (sda) will not be processed.

While that might be a solution to your problem, your question is very interesting. There seems to be no eval feature in Jinja2. While you can concatenate strings you can not use that string as a variable name to get to its value...

Your tasks can be as simple as this...

- stat:
    path: /dev/{{item}}1
  with_items: ansible_devices.keys()
  when: item != 'sda'
  register: stats
- command: /sbin/parted -s /dev/{{item.item}} mklabel gpt
  with_items: stats.results
  when: item.stat | default(false) and item.stat.exists