(for GFv2)
As I recall it asks for an admin name and admin password when you install GF.
Nevertheless, I believe you can simply go to the admin-realm in the console and add a new user (ladmin) and then delete the existing admin user.
[Message sent by forum member 'whartung' (whartung)]
http://forums.java.net/jive/thread.jspa?messageID=327734