You can set a ProxyCommand in the ansible_ssh_common_args inventory variable. Any arguments specified in this variable are added to the sftp/scp/ssh command line when connecting to the relevant host(s).
You can create group_vars/gatewayed.yml with the following contents:
ansible_ssh_common_args: '-o ProxyCommand="ssh -W %h:%p -q firstname.lastname@example.org"'
Ansible will append these arguments to the command line when trying to connect to any hosts in the group gatewayed.