Issues installing GM on both Windows and Mac. Error 1013.
have you checked your hosts file? tiny umbrella adds a reference for gs.apple.com every time you use it and therefore wont authorise the firmware installation, not sure if thats same thing but ive read 1004 is the error you get for currently signed firmware (ie 4.1) and 1013 for beta or non current official builds.
If it is in there delete the refernce or stick a # at the beggining to comment the line out.