The method edit_group_hook uses POST, but the API requires PUT: https://docs.gitlab.com/api/group_webhooks/#edit-group-hook. A POST request generated a 404 error. See https://github.com/NARKOZ/gitlab/blob/8aef58f39fb4c3a8e568bba8ad1c6d131abfead0/lib/gitlab/client/groups.rb#L456.