Geant4 home:
http://geant4.web.cern.ch/ (http://www.geant4.org)

User documentation:
http://geant4.web.cern.ch/support/user_documentation

Reference guide:
http://www-geant4.kek.jp/Reference/

User forum:
http://geant4-forum.web.cern.ch

GPS examples: 
http://hurel.hanyang.ac.kr/Geant4/Geant4_GPS/reat.space.qinetiq.com/gps/examples/examples.html

My web page:
http://www.fe.infn.it/u/paterno/