This upload fixes the issue:
gdm (2.14.0-0ubuntu4) dapper; urgency=low
.
* debian/control:
- Depends on "| xubuntu-default-settings" for xubuntu (Ubuntu: #38551)
** Changed in: gdm (Ubuntu)
Status: Confirmed => Fix Released
--
gdm should depends on xubuntu-artwork too
https://launchpad.net/malone/bugs/38551