That's fixed with that upload:
gdm (2.17.6-0ubuntu2) feisty; urgency=low
.
* debian/rules:
- build with --enable-ipv6=yes
** Changed in: gdm (Ubuntu)
Status: Confirmed => Fix Released
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XDMCP not working in ipv6, gdm should be compiled with --enable-ipv6=yes
https://launchpad.net/bugs/75254