ALKIS/ATKIS - German National Cadastre
ALKIS® = Authoritative Real Estate Cadastre Information System
There is English documentation for this new German standard which also explains how the mapping to GML is done.
The XML schema files can be found on the German version of that webpage, which is much more comprehensive: http://www.adv-online.de/extdeu/broker.jsp?uMen=4b370024-769d-8801-e1f3-351ec0023010
Here is a direct link to the ZIP-File with the GML schemas: http://www.adv-online.de/extdeu/binarywriterservlet?imgUid=9815019a-8c61-b111-a3b2-1718a438ad1b&uBasVariant=22222222-2222-2222-2222-222222222222
Testdatasets can be found following the links from the following webpage (sorry, only in German): http://www.lv-bw.de/alkis.info/nas-bsp.html