gistfile1.txt

cb 1 (.venv) ~/git/test-playbook
$ ansible-playbook test.yml -i inventory/hosts

PLAY [test] *******************************************************************

GATHERING FACTS ***************************************************************
ok: [localhost]

TASK: [debug ] ****************************************************************
ok: [localhost] => (item=hostvars[groups['g1'][0]]['y']) => {
    "hostvars[groups['g1'][0]]['y']": "{{x}}",
    "item": "hostvars[groups['g1'][0]]['y']"
}
ok: [localhost] => (item=y) => {
    "item": "y",
    "y": "foo"
}

PLAY RECAP ********************************************************************
localhost                  : ok=2    changed=0    unreachable=0    failed=0

cb 1 (.venv) ~/git/test-playbook
$ cat inventory/hosts
[all]
localhost

[g1]
localhost
cb 1 (.venv) ~/git/test-playbook
$ cat inventory/group_vars/g1.yml
---
x: foo
y: "{{x}}"
cb 1 (.venv) ~/git/test-playbook
$ cat test.yml
---
- name: test
  hosts: localhost
  tasks:
    - debug:
        var: "{{item}}"
      with_items:
      - hostvars[groups['g1'][0]]['y']
      - y