If you really want to append to content, you will need to use the set_fact module. But if you just want to use the merged lists it is as easy as this:

{{ list1 + list2 }}

With set_fact it would look like this:

- set_fact:
    list_merged: "{{ list1 + list2 }}"

NOTE: If you need to do additional operations on the concatenated lists be sure to group them like so:

- set_fact:
    list_merged: "{{ (list1 + list2) | ... }}"

    - set_fact:
        items_list: "{{ items_list | default(['item1']) + [item] }}"
      loop:
        - "{{ (True and True) | ternary('item2', None) }}"
        - "{{ (True) | ternary('item3', None) }}"
      when: item
    - debug: var=items_list

or

     - set_fact:
         items_list: "{{ items_list | default(['item1']) + [item] }}"
       loop:
         - "item2"
         - "item3"
       when: >-
         (item == "item2" and True and True) or
         (item == "item3" and True)
     - debug: var=items_list

You can merge two lists in a variable with +. Say you have a group_vars file with this content:

---
# group_vars/all
pgsql_extensions:
  - ext1
  - ext2
  - ext3

And it's used in a template pgsql.conf.j2 like:

# {{ ansible_managed }}
pgsql_extensions={% for item in pgsql_extensions %}{{ item }}, {% endfor %}

You can then append extensions to the testing database servers like this:

---
# group_vars/testing_db
append_exts:
  - ext4
  - ext5
pgsql_extensions: "{{ pgsql_extensions + append_exts }}"

When the role is run in any of the testing servers, the aditional extensions will be added.

I'm not sure this works for dictionaries as well, and also be careful with spaces and leaving a dangling comma at the end of the line.

You can do it like this:

- name: create new list for tunnel
  set_fact:
    list_tunnel: "{{ list_tunnel | default([]) + ['tunnel.' + item | string] }}"
  loop: "{{ range(1,10) | list}}"

- debug:
    msg: "{{ list_tunnel }}"

or

- set_fact:
    list_tunnel: "{{ result }}"
  vars:
    prefix: tunnel
    a_list: "{{ range(1, 10) | list }}"
    result: "{{ [prefix] | product(a_list) | map('join', '.') | list }}"

Q: "Prepend a string 'delete' in front of each item."

A: Is this what you're looking for?

- hosts: localhost
  vars:
    info: ['a', 'b', 'c']
  tasks:
    - set_fact:
        payload: "{{ payload|default([]) + ['delete ' ~ item] }}"
      loop: "{{ info }}"
    - debug:
        var: payload

gives (abridged)

payload:
  - delete a
  - delete b
  - delete c

The same result can be achieved without iteration (credit @Romain DEQUIDT)

- hosts: localhost
  vars:
    info: ['a', 'b', 'c']
    payload: "{{ ['delete']|product(info)|map('join', ' ')|list }}"
  tasks:
    - debug:
        var: payload

Q: "Prepend a string 'delete' in front of each output that starts with 'gtm topology'."

A: Use the test search

- hosts: localhost
  gather_facts: false
  vars:
    info: ['a', 'gtm topology', 'c']
  tasks:
    - set_fact:
        payload: "{{ payload|default([]) + ['delete ' ~ item] }}"
      loop: "{{ info }}"
      when: item is search('^gtm topology')
    - debug:
        var: payload

gives (abridged)

payload:
  - delete gtm topology

The same result can be achieved without iteration

- hosts: localhost
  vars:
    info: ['a', 'gtm topology', 'c']
    info_gtm_topology: "{{ info|select('search', '^gtm topology') }}"
    payload: "{{ ['delete']|product(info_gtm_topology)|map('join', ' ')|list }}"
  tasks:
    - debug:
        var: payload

Q: "In addition to the conditions above, process the list till 'test-pool1' is found."

A: Use the test search also to find the index of the last item

  hosts: localhost
  vars:
    info: ['a', 'gtm topology 1', 'c', 'test-pool1', 'gtm topology 2', 'd']
    stop_regex: '.*pool1.*'
    stop_items: "{{ info|select('search', stop_regex) }}"
    stop_index: "{{ info.index(stop_items.0) }}"
  tasks:
    - set_fact:
        payload: "{{ payload|default([]) + ['delete ' ~ item] }}"
      loop: "{{ info[:stop_index|int] }}"
      when: item is search('^gtm topology')
    - debug:
        var: payload

gives (abridged)

payload:
  - delete gtm topology 1

The same result can be achieved without iteration

- hosts: localhost
  vars:
    info: ['a', 'gtm topology 1', 'c', 'test-pool1', 'gtm topology 2', 'd']
    stop_regex: '.*pool1.*'
    stop_items: "{{ info|select('search', stop_regex) }}"
    stop_index: "{{ info.index(stop_items.0) }}"
    info_gtm_topology: "{{ info[:stop_index|int]|select('search', '^gtm topology') }}"
    payload: "{{ ['delete']|product(info_gtm_topology)|map('join', ' ')|list }}"
  tasks:
    - debug:
        var: payload

shell> cat filter_plugins/list_search.py 
import re


def list_search(l, x):
    r = re.compile(x)
    return list(filter(r.match, l))


def list_index(l, x, *i):
    if len(i) == 0:
        return l.index(x) if x in l else -1
    elif len(i) == 1:
        return l.index(x, i[0]) if x in l[i[0]:] else -1
    else:
        return l.index(x, i[0], i[1]) if x in l[i[0]:i[1]] else -1


class FilterModule(object):
    ''' Ansible filters for operating on lists '''

    def filters(self):
        return {
            'list_index': list_index,
            'list_search': list_search,
        }

- hosts: localhost
  vars:
    info: ['a', 'gtm topology 1', 'c', 'test-pool1', 'gtm topology 2', 'd']
    stop_regex: '.*pool1.*'
    stop_items: "{{ info|list_search(stop_regex) }}"
    stop_index: "{{ info|list_index(stop_items.0) }}"
    info_gtm_topology: "{{ info[:stop_index|int]|select('search', '^gtm topology') }}"
    payload: "{{ ['delete']|product(info_gtm_topology)|map('join', ' ')|list }}"
  tasks:
    - debug:
        var: payload

payload:
  - delete gtm topology 1

adding an answer to close the thread, with the official response i got from the Ansible team on the issue i opened at Github. Obviously this was already known to them, they closed my issue as duplicate redirecting me to these 2:

#38302 and #38075