alias gssh='gcloud compute ssh --tunnel-through-iap'

_gssh() {
  local cur=${COMP_WORDS[COMP_CWORD]}
  [[ "$cur" = -* ]] && return 0
  local hosts=$(gcloud compute instances list --format='value(name)')
  COMPREPLY=( $(compgen -W "$hosts" -- "$cur") )
  return 0
}

complete -F _gssh gssh