try this
http://www.redmondpie.com/fix-itunes-error...4-to-ios-4.2.1/

i did this today when updating my iphone 4 to 4.2.1 from 4.1

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Added on March 5, 2011, 3:14 amopps sorry wrong post forget to see that u did what the link told you...i myself get that gs.apple.com in my host..but even i deleted it also still have that error until i restart my itunes and replug in my iphone

and possible u can download the firmware from the web instead of using itunes? maybe is a bad firmware u got there?

This post has been edited by intothefantasy: Mar 5 2011, 03:17 AM