Simple zsh completion for insync, insync-headless and insync-portable. Grab this, name it "_insync-headless" and put it in some suitable location for zsh completion functions. And good luck. Zsh completion is a mystery to me.

_insync-headless

#compdef insync-headless insync-portable insync

if [[ $CURRENT == 2 ]]
then
   _values 'commands' \
      'accept_all_new_shares[Accept  all  new   shares   file  to  given  account]' \
      'accept_share[Accept shared file to given account]' \
      'add_account[Add account to Insync]' \
      'force_sync[Force sync item]' \
      'get_account_information[Show information for connected acounts]' \
      'get_actions_required[Show required actions]' \
      'get_domain_link[Get domain link for file]' \
      'get_errors[Show error messages]' \
      'get_file_status[Get syncing status of file]' \
      'get_new_shares[Show new shares]' \
      'get_private_link[Get private link for file]' \
      'get_proxy[Show proxy configuration]' \
      'get_public_link[Get public link for file]' \' \
      'get_recent_changes[20 most recent changes]' \
      'get_root_drive_files[Get selective sync information for root Drive files and folders of account]' \
      'get_status[Show syncing status]' \
      'get_sync_progress[Show progress of files being downloaded/uploaded]' \
      'manage_selective_sync[Manage selective sync settings of account]' \
      'move_folder[Move folder to a new path]' \
      'open_in_gdrive[Open selected file in Google Drive web interface]' \
      'pause_syncing[Pause syncing]' \
      'quit[Quit]' \
      'reject_all_new_shares[Reject  all  new   shares   to  given  account]' \
      'reject_share[Reject shared file to given account]' \
      'remove_account[Remove account from Insync]' \
      'remove_from_insync[Remove linked file/folder from Insync]' \
      'resume_syncing[Resume syncing]' \
      'retry_errors[Retry all errors]' \
      'set_autostart[Set autostart preference (yes/no)]' \
      'set_export_option[Download Google Docs files as docs or links]' \
      'set_proxy[Set proxy config]' \
      'set_selective_sync[Set selective sync preference (yes/no) of file for given email]' \
      'share[Share selected file using Insync web interface]' \
      'start[Start Insync]'
   return
fi

# we are completing the second or a subsequent argument.

local insync='insync'

which $insync > /dev/null || insync='insync-headless'
which $insync > /dev/null || insync='insync-portable'

if ! which $insync > /dev/null
then
   # Provide some kind of (possibly useless) default.
   _path_files
   return
fi

__insync_get_all_emails ()
{
   local -a accounts
   accounts=( $( $insync get_account_information 2> /dev/null | grep -E -o "\<[a-zA-Z0-9.-]+@[a-zA-Z0-9.-]+\.[a-zA-Z0-9.-]+\>" ) )
   (( 0 < $#accounts )) && _values "accounts" $accounts
}

if (( 2 < $CURRENT ))
then
   case $words[2] in
      'get_root_drive_files'  ) [[ $CURRENT == 3 ]] && __insync_get_all_emails ;;
      'remove_account'        ) [[ $CURRENT == 3 ]] && __insync_get_all_emails ;;
      'manage_selective_sync' ) [[ $CURRENT == 3 ]] && __insync_get_all_emails ;;
      'accept_all_new_shares' ) [[ $CURRENT == 3 ]] && __insync_get_all_emails ;;
      'reject_all_new_shares' ) [[ $CURRENT == 3 ]] && __insync_get_all_emails ;;
      'accept_share'          ) [[ $CURRENT == 4 ]] && __insync_get_all_emails ;;
      'reject_share'          ) [[ $CURRENT == 4 ]] && __insync_get_all_emails ;;
      'add_account'           ) [[ $CURRENT == 4 ]] && _path_files -/ ;;
      'force_sync'            ) [[ $CURRENT == 3 ]] && _path_files -/ ;;
      'get_file_status'       ) [[ $CURRENT == 3 ]] && _path_files -f ;;
      'get_private_link'      ) [[ $CURRENT == 3 ]] && _path_files -f ;;
      'get_public_link'       ) [[ $CURRENT == 3 ]] && _path_files -f ;;
      'open_in_gdrive'        ) [[ $CURRENT == 3 ]] && _path_files -f ;;
      'share'                 ) [[ $CURRENT == 3 ]] && _path_files -f ;;
      'remove_from_insync'    ) [[ $CURRENT == 3 ]] && _path_files ;;
      'get_domain_link'       ) [[ $CURRENT == 3 ]] && _path_files ;;
      'set_autostart'         ) [[ $CURRENT == 3 ]] && _values "autostart" "yes" "no" ;;

      'set_selective_sync'    ) [[ $CURRENT == 3 ]] && __insync_get_all_emails
                                [[ $CURRENT == 5 ]] && _values "selective sync" "yes" "no" ;;

      'move_folder'           ) [[ $CURRENT == 3 ]] && _path_files -/
                                [[ $CURRENT == 4 ]] && _path_files -/ ;;

      'set_export_option'     ) [[ $CURRENT == 3 ]] && __insync_get_all_emails
                                [[ $CURRENT == 4 ]] && _values "document export format" "office" "link" ;;
   esac
fi

For anyone wondering where to put this, create the file in a folder (like ~/..zsh) and then add this to your ~/.zshrc file:
fpath=(~/.zsh/ $fpath)

So that it gets picked up. Make sure it begins with an underscore!

Thanks for making this, btw.